Nobody sets out to build a diluted version of their own design. Yet almost every homeowner I meet at handover says a version of the same sentence: it’s lovely, but it isn’t quite what I pictured.
I’ve spent years on London sites as a main contractor, and I can tell you the reason is almost never the design. The mood board was good. The drawings were good. The taste was never in question. What went missing sat in a narrower, duller place — the specification. The unglamorous middle layer betThe Specification Gap: Why the Finished Room Never Quite Matches the Designween “I want a seamless poured floor” and the forty separate build decisions that determine whether that floor is flat, warm, dry, crack-free and sitting at the right height against your doors.
That gap is where interiors quietly lose their edge. It isn’t a dramatic failure. It might be a threshold sitting 18mm higher than planned, forcing a flooring revision. A radiator may return to a carefully cleared wall because the heating system cannot operate at the temperature the original design required. A fine crack could appear across a poured floor months later, while a sleek steel screen might develop persistent condensation on cold winter mornings.
Each one is small. Together they are the difference between the room you designed and the room you got.
This article is the technical half of the conversation — the half interiors writing usually skips because it involves millimetres and British Standards rather than palettes. I’ve organised it as the seven stages where design intent actually leaks out, in the order they happen on a real project. Every figure quoted is a real published requirement with its source named, so you can take any line here and put it directly to whoever is building for you.
What the specification gap actually is
A design is a description of an outcome. A specification is a description of how that outcome gets built — the products, the thicknesses, the tolerances, the sequence.
Most homeowners arrive with an excellent design and no specification. That isn’t a criticism; it’s normal. The problem is that the gap gets filled anyway. Somebody has to decide what insulation goes under that floor, what U-value the doors hit, how thick the screed is, where the movement joints fall. If you haven’t decided, the decision still gets made — on site, at speed, by whoever needed an answer that morning, weighted toward whatever is quickest and already on the van.
Design intent leaks out in exactly three places, and it’s worth naming them before we go further.
It leaks at survey.
Something about the existing building was never measured, so the design was drawn against an assumption that turned out to be wrong. Floor levels, water pressure, ground moisture, wall build-up. This is the most expensive leak because it’s discovered latest.
It leaks in the pricing.
Two builders price the same drawings. One prices what’s drawn; one prices a cheaper way of achieving something that photographs the same. You compare totals, choose, and inherit the difference without ever having seen it.
It leaks at first fix.
The window between “walls open” and “walls closed” is where the permanent decisions get made — heating, insulation, waterproofing, fire protection, floor build-up. It’s short, it’s noisy, it happens while you’re distracted by tile samples, and everything decided in it is invisible forever after.
Close those three and your design arrives more or less intact. Leave them open and no amount of good taste rescues it.
Stage one — the survey nobody pays for, and the three things it must measure
The cheapest hour of any project is the one spent measuring the building before the design is fixed. It is also the hour most often skipped, because at that point nobody has been paid and everybody is optimistic.
Three measurements change the design if they come back badly. Take these three and you have removed most of the risk of a late, ugly redesign.
Water pressure and flow
This is the one I’d put first, and interiors people find that surprising. But consider what a modern design-led bathroom assumes: a large rain head, a second handset, sometimes a bath filling at the same time, sometimes two bathrooms running at once. All of that is a flow-rate demand, and flow rate is a property of the building, not of the taps you choose.
Here’s the part worth knowing. Your water company is held to a guaranteed standard of 7 metres static head — roughly 0.7 bar — in the communication pipe, under the Ofwat Guaranteed Standards Scheme and the Water Industry Act 1991. Ofwat’s reference level of service, the old DG2 measure, is 10 metres head at a flow of 9 litres per minute at the boundary stop tap; below that a property is formally counted as receiving low pressure.
Read those numbers carefully, because there’s a trap in them. Both are measured at the boundary. There is no obligation on pressure inside your own private supply pipe or inside the property. So a house can be entirely within standard at the stop tap and still deliver a disappointing shower, because the run from the boundary to your bathroom is old, narrow, partly lead, or shared.
The design consequence is direct. If flow is limited, you are choosing between an unvented cylinder, a pump and accumulator, or a smaller specification of outlets. An unvented hot water system is notifiable work with required safety devices and a correctly terminated discharge pipe under Approved Document G, §3.22–3.23, and it needs a cylinder cupboard somewhere. That cupboard has to come out of a room. If you discover this after the joinery is drawn, the joinery changes.
Ten minutes with a pressure gauge and a measured jug at the start prevents that. It’s worth reading the detail on how to test the water pressure before anything is designed, because the test itself is simple enough to do yourself before you’ve spent a penny.
One more figure while we’re here, because it constrains fittings on new dwellings: potential consumption of wholesome water must not exceed 125 litres per person per day, or 110 litres where planning has imposed the optional tighter requirement (Building Regulations 2010, reg. 36 / Approved Document G2). That is a real limit on how extravagant a shower specification can be on a new build.
Floor level, flatness and deflection
Every seamless floor finish, every large-format tile, every flush threshold and every full-height door depends on the floor being flatter and stiffer than the existing floor probably is.
The governing figure is deflection. A substrate receiving a rigid finish should deflect no more than span ÷ 360 under imposed load, and span ÷ 720 where natural stone is used (BS 5385-3 and adhesive manufacturers’ technical guidance). A timber floor that flexes more than that will crack a rigid finish laid on it — not immediately, but reliably.
In older housing stock, particularly Victorian and Edwardian terraces, that check frequently fails. Joists have been notched by decades of plumbers, spans are long, and the floor has a gentle bounce that nobody notices when walking on carpet. Lay a rigid finish on it and you have built in a crack.
Measure it before you fall in love with a finish, not after. If the floor fails the deflection test the honest answers are: stiffen it (sister the joists, add noggins, reduce the effective span), change to a finish that tolerates movement, or accept a floating build-up — and each of those changes the finished floor height, which changes every threshold and door in the room.
Ground moisture
If you are working on a ground floor, particularly an older solid floor, establish whether there is a damp-proof membrane before you specify anything that sits on it.
A floor next to the ground must resist the passage of moisture into the building — that’s Requirement C2 of the Building Regulations, supported by Approved Document C. An existing solid floor with no membrane will push moisture upward continuously into whatever you lay on it. It doesn’t matter how good the product is. Moisture drive from below defeats adhesives, lifts resins, and blooms through cement-based finishes.
Test it. Then specify against the answer.
Stage two — how a beautiful scheme gets priced away
You’ve done the work. The scheme is drawn, the finishes are chosen, the survey came back. You send it to three contractors and the prices come back materially apart.
The instinct is to ask why the expensive one is expensive. The better question, and the one that protects your design, is: what is inside each price?
Let me give you the example I use on site, because it makes the abstract concrete.
A pitched roof has dozens of timber-to-timber connections. There are two honest ways to make each one. You can skew-nail the joint with a nail gun — a few nails, one pass, done. It works on day one, the roof looks perfect, the tiles go on. Or you can fit a galvanised joist hanger at the joint with a purpose-made square-twist sherardised nail in every single hole — hangers to BS EN 845-1, nails typically 30 × 3.75mm.
Here is the detail that decides it. A joist hanger’s published load capacity assumes every hole is filled with the specified fixing. Fill fewer holes, or substitute ordinary round-wire nails from the gun because they’re already loaded, and the connection’s capacity falls by roughly 50–75% (manufacturer installation data / BS 1202 nail specification).
From above, the two roofs are the same roof. They photograph identically. The difference doesn’t appear on day one — it appears in year three, as cracks at ceiling junctions, or movement nobody can explain.
That is the shape of every price difference I have ever examined. It is almost never a bigger margin. It is specification depth — how precisely the quote says what will be built, and with what.
For a design-led project, three tests will tell you most of what you need.
Is the scope actually identical?
The most common reason one quote is lower is that it’s a smaller job wearing the same name. Put both scopes side by side, line by line, and mark what is missing rather than what is different. Missing scope reappears later as a variation, at a price you no longer have leverage over.
Are materials named?
“Insulation” is not a specification. A named product, a thickness, and a performance figure is. “Feature lighting” is not a specification. Unnamed materials are a licence to substitute downwards after you’ve signed — and the substitution will always be toward the cheaper equivalent, never the dearer one.
Are the invisible trades priced at all? Fire-stopping, cavity trays, wall ties, dry-packing, tanking, padstones, movement joints. None of these appear on a mood board. All of them cost money and time. A quote that omits them isn’t cheaper; it’s incomplete, and the gap will find you.
If you want the long version of this, including what a properly built quote looks like line by line, it’s set out in detail here: what actually sits inside two different prices.
One last point on pricing, and it matters more to design-led clients than to anyone else. A quote full of provisional sums for the very things that define your scheme — the joinery, the stone, the glazing, the lighting — isn’t a price. It’s a placeholder. Those are precisely the items you should have nailed to a named product and a real number before signing, because they are the items your design lives in.
Stage three — the heating decision that quietly redesigns your rooms
This is the one that catches design-led clients hardest, because it looks like a services decision and it’s actually a spatial one.
Approved Document L (Volume 1, 2021) changed the calculation. Where a wet heating system is newly installed, or fully replaced in an existing building — appliance, emitters and pipework — all parts should be sized to operate effectively at a maximum flow temperature of 55°C or lower (§5.10).
Sit with that for a moment. A traditional radiator system was sized around much hotter water. Drop the flow temperature to 55°C and each emitter delivers less heat, so to heat the same room you need more emitter surface. In practice: bigger radiators, or more of them, or underfloor heating.
That is a design decision dressed as a plumbing one. The wall you were keeping clear for artwork, the run of low joinery under the window, the clean corner — those are the things that get eaten when the radiator schedule is recalculated late.
Underfloor heating is usually the design answer. But it brings its own dimensional consequences, and this is where floor heights start moving.
- Downward heat loss. Ground floors and floors in contact with the outside should be insulated to limit heat loss to not more than 10 W/m² (§6.29).
- Insulation under intermittent systems. Underfloor heating intended for intermittent or cyclical operation, or installed over unheated rooms, should be separated from the structural floor by insulation with a thermal resistance of at least 1.25 m²K/W (§6.30).
- Intermediate floors. Per BS EN 1264-4, not less than 0.5 m²K/W for electric systems and 0.75 m²K/W for wet systems (§6.31).
- Electric cable screed depths. Direct electric systems sit within screeds not exceeding 60mm. Night energy-storage systems need screeds of at least 65mm (§6.33).
- Controls. All underfloor systems need controls to adjust operating temperature; electric room thermostats need a manual override; systems in screeds thicker than 65mm should automatically set back at night or when unoccupied (§6.28). In new dwellings, and when a heat generator is replaced in an existing one, each room should have thermostatic room controls (§5.20).
Now add it up as a designer would. Insulation plus screed plus your chosen finish equals a floor build-up. That build-up sets your finished floor level. Finished floor level sets whether your doors still fit, whether the threshold to the garden is flush or stepped, whether the run from the old floor into the new extension is seamless or has a lip, and whether that lovely tall skirting still reads correctly against the reduced ceiling height.
I have watched a flush indoor–outdoor threshold — the single detail the client cared most about — disappear because the floor build-up was settled three months after the doors were ordered. It cannot be recovered afterwards without lifting the floor.
Decide heating before you fix floor levels. Not after. The full comparison of the two systems, including running costs and where each genuinely belongs, is here: electric against wet underfloor heating, compared honestly.
Stage four — glazing, and the difference between a warm profile and a cold one
Slim steel glazing is the defining look of the last decade of interiors. Crittall-style screens, black frames, thin sightlines, that grid. It is genuinely beautiful and I fit a lot of it.
It also has a failure mode that is entirely predictable and entirely preventable, and the deciding phrase is thermally broken.
Steel conducts heat efficiently. A slim steel frame with no thermal break creates a continuous cold path from outside to inside. In a British winter, the internal face of that frame drops below the dew point of the room air, and water condenses on it — every cold morning, running down onto the sill, into the reveal, eventually into whatever finish you chose. The regulations are explicit that junctions and openings must be designed to avoid surface condensation and mould growth (Requirement C2 / Approved Document C), and an unbroken metal profile on an external wall is close to a textbook case.
The performance requirements are specific, and they’re worth knowing because suppliers do not always volunteer them.
- New and replacement doors in existing dwellings: maximum U-value 1.4 W/(m²·K), or Doorset Energy Rating Band B minimum. Doors with more than 60% of the internal face glazed: 1.4 W/(m²·K) or DER Band C minimum (Approved Document L Vol 1 2021, Table 4.2).
- New and replacement windows in existing dwellings: maximum 1.4 W/(m²·K), or Window Energy Rating Band B minimum (same table).
- No worse than what it replaces. A replacement element’s U-value must be no worse than the element being replaced, as well as meeting the Table 4.2 limits (§4.8).
- Extensions must meet the same Table 4.2 standards (§4.9a).
- Heritage exception. Where character must be maintained: either a centre pane U-value not exceeding 1.2 W/(m²·K), or single glazing supplemented with low-emissivity secondary glazing (§4.10).
The practical guidance I give every client is simple. Cold profile is for internal screens only. Internally there is no temperature difference across the frame, so there’s no condensation risk, and you get the slimmest, most authentic sightline available — use it freely for a study partition or an internal door set. Anything external goes warm profile, thermally broken, every time. The sightline penalty is marginal; the difference in how the room behaves in February is not.
That distinction — where cold profile is right and where it’s a mistake — is set out with the comparison in full here: slim steel and Crittall-style doors, warm profile against cold.
Stage five — seamless floors, and why they crack
The poured, jointless, no-tiles floor is the other signature finish of the current decade. Microcement, macrocement, resin. It’s on every mood board I’m handed.
It is also the finish I am asked to repair most often, and when I investigate, the product is rarely the villain. Three things cause the crack, and all three are decided before a trowel is lifted.
The substrate moves.
We covered the number in stage one: deflection no more than span ÷ 360, or span ÷ 720 under natural stone (BS 5385-3). A floor that flexes more will crack a rigid finish. A cement-based seamless finish is rigid. The maths is unforgiving.
The movement joints were ignored.
Movement joints are required at perimeters and over structural joints, with intermediate joints on internal floors at the spacings given in the code (BS 5385-3). A seamless finish must respect the joints in the structure beneath it. “Seamless” describes the appearance, not the physics. Where the structure moves, the finish must be allowed to move, or it will make its own joint — and it will choose the location.
The screed was still wet.
This is the most common of the three and the most avoidable. The drying guidance is one day per millimetre of thickness for the first 50mm, then roughly two days per millimetre between 50mm and 75mm (BS 8204-1 / BS 8203). A 75mm screed is therefore not a fortnight. It’s around 100 days.
And the calendar is not the test. Confirm with a surface hygrometer reading before anything goes down. I have never regretted insisting on a hygrometer reading, and I have several times regretted accepting somebody’s confident assurance that “it’s been ages.”
There’s one further distinction that matters enormously in a bathroom, and it is regularly got wrong. Where the finish forms part of a shower or wet room, the waterproof layer must be continuous (Requirement C2). A cement-based finish is not itself a tanking system. A waterproof resin system is. If a supplier tells you microcement will waterproof your wet room, that is not correct — it needs a tanking layer beneath it, and the tanking is the waterproofing, not the finish.
The full comparison — which family cracks, which flexes, which is genuinely waterproof, and where each belongs — is set out here: microcement, macrocement and resin compared properly.
There is a bonus in this section for anyone renovating a bathroom on a tight programme. If the existing floor is level and the existing tiles are sound and well-bonded, a resin system can sometimes go straight over the top — no strip-out, no skip, no fortnight of dust. It doesn’t always work, and the conditions have to be checked properly, but where it does work it turns a three-week job into a much shorter one.
Stage six — everything that disappears behind plasterboard
There is a specific afternoon on every project when the plasterboard goes up. Before it, everything is inspectable. After it, nothing is — not without cutting a hole and admitting you should have looked earlier.
This is the highest-stakes moment in the whole job and it usually passes without comment, because from the client’s point of view nothing visible happened. Here is what to look at before that afternoon.
Fire-stopping
The rule is broad and absolute: every joint, imperfect fit and opening for services through a fire-separating element should be sealed with fire-stopping so that the fire resistance of the element is not impaired (Approved Document B Vol 1, §9.1).
Homeowners tend to assume this is a subject for blocks of flats. On a standard terraced house it comes up constantly. A loft conversion adds a third storey, which triggers a protected escape route — the stair enclosure becomes fire-separating construction, and every cable, pipe and downlight crossing it needs treating. A flat conversion has compartment walls and floors between dwellings. Any of it, done badly, is invisible the moment the board goes on.
A pipe passing through a fire-separating element must use one of three permitted routes: a proprietary tested sealing system at any diameter; fire-stopping around a pipe of restricted diameter per Table 9.1; or a sleeving detail (§9.2–9.5). The diameter limits between flats or dwellinghouses are 160mm for high-melting-point metal; 160mm stack / 110mm branch for uPVC, lead, aluminium or fibre-cement in an enclosure to Diagram 9.1; and 40mm for any other material. In any other situation non-metal pipes drop to 40mm. The sleeving alternative applies up to a 160mm maximum nominal internal diameter, sleeved in a metal that will not soften or fracture at 800°C (§9.5).
There is also a handover duty that almost nobody claims: fire safety information must be given to the building owner at completion (Building Regulations 2010, Regulation 38). Ask for it. The full explanation of what to look for and when to ask is here: fire-stopping, and how to check yours was done.
The damp details
Damp is very rarely bad luck. Two details decide it, and both are checkable with your own eyes before anything is closed up.
A damp-proof course on an external wall should be at least 150mm above the level of the adjoining ground (Approved Document C, §5.5b). Raise the patio, lay a new deck, build up the garden — and if that 150mm is lost, you have bridged the damp-proof course, and moisture will come through regardless of how good the wall is. I see this constantly on projects where the landscaping was done by someone else, later, with the best of intentions.
On an external cavity wall, the cavity should be taken at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray provided to stop water passing into the inner leaf (§5.5c).
And then the weep holes. Where a damp-proof tray is used, provide weep holes every 900mm. Where the tray doesn’t extend the full length of the exposed wall — which is the case above every window and door — provide stop ends and at least two weep holes (§5.5c, Diagram 9b).
Those small open joints at the base of a wall, or in the course above a window, are not sloppy bricklaying. They are the drain. A cavity wall works by accepting that some water gets through the outer leaf, catching it on a tray, and leading it back out through those holes. Fill them in because they look untidy and you have sealed the drain. The detail is worth understanding properly: the damp-proof course and the weep holes.
Wall ties and lintel bearings
Two more numbers, because they explain most of the cracks people ask me about.
Wall ties must comply with BS EN 845-1 (austenitic stainless steel, material reference 1 or 3), at 900mm horizontal × 450mm vertical spacing or a minimum of 2.5 ties/m², with additional ties at not more than 300mm vertically within 225mm of openings, movement joints and roof verges (Approved Document A, §2C8 and §2C19).
Lintel bearing is the one behind those diagonal cracks climbing from the corner of a window. Vertical loading may be assumed to be distributed where the bearing length is 150mm or greater. Where a lintel has a clear span of 1200mm or less, the bearing may be reduced to 100mm (Approved Document A, §2C24a). Beyond that — and for any beam replacing a loadbearing wall, which is most open-plan schemes — the design belongs to a structural engineer, including bearing length, padstone size and beam section. Many lintel manufacturers require a greater minimum bearing, commonly 200mm, on longer spans; follow the manufacturer’s published data as well as the Approved Document.
And a subtlety that matters in older houses: you can have a perfectly correct 150mm bearing and still crack, if the masonry underneath cannot carry the load. The end of a bearing quite often lands on soft, weathered or previously repaired brick. Where that’s the case, the weak bricks are cut out and replaced with engineering brick — Class B is at least 75 N/mm² compressive strength with water absorption below 7%; Class A is at least 125 N/mm² with absorption below 4.5% (BS EN 771-1, UK National Annex).
You will not be checking wall tie spacing yourself. But asking the question tells the person building for you that somebody is paying attention, and that alone changes behaviour on site more than any clause in a contract.
A note on all the figures above: Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.
Stage seven — who is actually holding your design
Everything above assumes somebody on the project is answerable for the design surviving. Frequently nobody is, and that is a structural problem rather than a personal one.
The traditional arrangement splits the job: a designer produces the scheme, a contractor prices and builds it. It works well when the drawings are complete and the specification is deep. It works badly when they aren’t, because the gap between “what was drawn” and “what was buildable” becomes a space where responsibility disappears. The designer says the contractor departed from the drawing. The contractor says the drawing couldn’t be built as shown. Both are often telling the truth, and neither is fixing your room.
What closes it is not a contract clause. It’s a single point of answerability for the outcome, held by somebody who was in the room for the technical decisions as well as the aesthetic ones.
Some practical ways to establish that, whatever route you take.
Insist on a specification, not just drawings.
A named product, thickness and performance figure for every significant element. If your designer doesn’t produce one, ask your contractor to write one and have your designer approve it. Somebody must own the list.
Agree a substitution rule in writing before work starts.
Nothing on the specification changes without your written sign-off. Not “we’ll let you know” — written sign-off. When something genuinely can’t be sourced, you should be brought options, not shown an installed alternative.
Fix the decision dates, not just the build dates.
Every specification item needs a date by which it must be decided, working backwards from when it’s needed on site. Most late substitutions happen because a decision arrived after the point where it could be honoured.
Put the invisible stages in the programme as inspection points.
Prior to Plasterboard Installation. Before screed. Ahead of the Final Floor Finish. Before the ceiling closes. Written into the programme, they happen. Left informal, they don’t.
Ask for photographs of what gets covered.
Not a formality — a record. When a question arises in year three about what’s behind a wall, the photograph is the answer, and it costs nothing to take at the time.
The specification checklist — sixteen questions, in the order you should ask them
Take this to your next meeting. It’s ordered by project stage, which makes it useful rather than merely comprehensive.
Before the design is fixed
- What is the measured static pressure and flow rate at the incoming main — and does the bathroom specification fit inside it?
- Does the existing floor pass the deflection test for the finish I want (span ÷ 360, or span ÷ 720 under stone)?
- Is there a damp-proof membrane in the ground floor, and has moisture been tested rather than assumed?
- Is the damp-proof course at least 150mm above the adjoining ground — and will any landscaping change that?
Things to Check First
- Are the two quotes covering identical scope, line by line — and what is missing from the cheaper one?
- Is every significant material named, with thickness and performance figure, rather than described generically?
- Are the invisible trades priced — fire-stopping, cavity trays, movement joints, padstones, tanking?
- Which items are provisional sums, and can the ones that define the design be fixed to a real product now?
Before the floor build-up is set
- What flow temperature is the heating system designed to run at, and what does that do to emitter sizes?
- What is the complete floor build-up — insulation, screed, finish — and what finished floor level does it produce?
- Do the doors, thresholds and skirting details still work at that level?
- If underfloor heating, does the insulation meet the required thermal resistance for its type and location?
Before the walls close
- Has every service penetration through a fire-separating element been fire-stopped, and can I see it before boarding?
- Are the weep holes open, at 900mm centres, with stop ends and at least two weep holes above every opening?
- Has the screed been confirmed dry by hygrometer reading — not by calendar — before any finish goes down?
- Do I have photographs of every stage that’s about to be covered up?
Sixteen questions. None of them require you to be technical. All of them require somebody to give you a straight answer, and the quality of the answers will tell you more about who you’ve hired than any portfolio.
Frequently asked questions
Why does my finished room never look like the design?
Almost always because the specification was incomplete, not because the design was wrong. Where a design says “seamless poured floor”, the specification has to say which system, over what substrate, at what thickness, with movement joints where, over which insulation, at what finished floor level. If those aren’t decided in advance they get decided on site under time pressure, and the result drifts from the intent.
What’s the single most expensive thing to get wrong?
The floor build-up, because it’s determined by the heating decision and it sets every threshold, door height and skirting proportion in the room. It also cannot be changed after the fact without lifting the floor. Decide heating before you fix floor levels.
Why does microcement crack?
Rarely the product. Three causes, all decided before application: the substrate deflects more than span ÷ 360; the movement joints in the structure below weren’t respected; or the screed hadn’t dried. Screed drying is roughly one day per millimetre for the first 50mm, then about two days per millimetre from 50mm to 75mm — confirmed by hygrometer, not by the calendar.
Do I need thermally broken frames for Crittall-style doors?
Externally, yes. Steel conducts cold, and an unbroken profile will condense on its internal face through winter. Internal screens don’t need a thermal break because there’s no temperature difference across the frame — that’s where cold profile genuinely belongs. New and replacement doors in existing dwellings must reach a maximum U-value of 1.4 W/(m²·K), or Doorset Energy Rating Band B.
When is the last moment to check the things that get hidden?
Before plasterboard, before screed, and before the floor finish. Put those three as named inspection points in the programme rather than leaving them informal. After plasterboard, checking anything means cutting a hole.
Should I use one company for design and build, or keep them separate?
Both work. What matters is that one party is answerable for the outcome and was present for the technical decisions as well as the aesthetic ones. Separate parties work well with a deep specification and a written substitution rule; they work poorly without one, because the gap between drawing and build becomes a place where responsibility goes missing.
The short version
Your design is probably fine. The specification is where it goes.
Measure the building before you fix the design. Read quotes for what’s inside them rather than what’s on the bottom line. Settle heating before floor levels, because heating decides build-up and build-up decides how every threshold in the room reads. Understand that seamless finishes are a substrate decision, not a product decision. And be present, or have somebody present on your behalf, on the afternoon the plasterboard goes up.
Do those five things and the room you walk into at handover will be recognisably the one you drew. That’s the whole trick, and there’s nothing glamorous in it — which is exactly why it gets skipped.
Vladimir Castravet is the founder of 2VP (2V Projects Ltd), a London main contractor working on renovations, extensions and loft conversions across west and south-west London. 2VP publishes a free technical library for homeowners covering the build details behind design decisions, and every figure in it is checked against the Approved Documents or the governing British Standard.
