How to Calculate Build Costs for a Property Development (UK, 2026)

Of every number in a development appraisal, the build cost is the one most likely to sink you. Get the purchase price wrong and you can usually renegotiate. Get the GDV wrong and a rising market might bail you out. But underestimate your build cost by 20% on a project with a 20% margin, and you have just turned your entire profit into someone else's. With material prices having settled but labour still tight across most of the UK in 2026, knowing how to build up a realistic construction budget — before you exchange — is the single most valuable appraisal skill you can have.

This guide walks through how to estimate build costs properly: the cost-per-square-metre method, what the headline rate does and doesn't include, and a full worked example you can copy.

Start with the floor area, not the asking price

The most reliable way to estimate construction cost early in a deal is the cost per square metre (£/m²) method. You take the gross internal floor area (GIA) of the finished scheme and multiply it by an appropriate rate for the type of work.

GIA is measured to the internal face of the external walls, across every floor. If you are converting a 90m² house into a two-storey rear-extended property of 130m², it is the 130m² figure you build against — the finished area, not the area you bought.

Indicative all-in build rates for 2026 (England, outside central London) look roughly like this:

Type of work	Typical £/m²
Light refurbishment (cosmetic, kitchens/bathrooms)	£600–£1,100
Heavy refurbishment (rewire, replumb, structural)	£1,100–£1,900
Conversion (house to flats, commercial to resi)	£1,300–£2,200
New build (standard spec)	£1,800–£2,600
New build (high spec / constrained site)	£2,600–£3,400

These are deliberately broad. A two-bed flat fit-out in Sunderland and the same job in Surrey can differ by 40%. Use them as a sanity check, then refine with local quotes as the deal firms up.

Know what the headline rate leaves out

The single most common appraisal error is treating the £/m² rate as the total cost of getting the building finished. It isn't. A build rate covers the bricks-and-mortar construction. Sitting on top of it are four cost layers that the rate ignores:

• Professional fees — architect, structural engineer, quantity surveyor, planning consultant, party wall surveyor. Budget 8–13% of build cost depending on complexity.
• Contingency — money set aside for the things you cannot see yet. 10% is standard for experienced developers on a familiar build; 15% is sensible for a first project, an older property, or anything with groundworks.
• Statutory and site costs — building control, CIL/Section 106 where it applies, utility connections, skips, scaffolding, ground investigation, and asbestos surveys on anything built before 2000.
• VAT — see below. This one alone can move a budget by tens of thousands.

Don't get caught out by VAT

VAT treatment depends entirely on what you are building, and developers regularly get it wrong:

• New build residential is zero-rated — your contractor should not be charging you 20% VAT on the construction.
• Converting a non-residential building into dwellings (e.g. an office or shop into flats) usually qualifies for the reduced 5% rate.
• Converting a property that changes the number of dwellings (one house into two flats, or two flats back into one house) also typically attracts the 5% rate.
• Straight refurbishment of an existing dwelling is generally standard-rated at 20%.

If you assume 20% VAT on a conversion that actually qualifies for 5%, you will reject deals that work. Assume 5% on a refurb that is really 20%, and you will buy a loss-maker. When in doubt, get it confirmed by a VAT-aware accountant before you commit — it is cheap advice relative to the sums involved.

A worked example: house to two flats

You are converting a tired three-bed semi into two self-contained flats. The existing GIA is 95m²; a modest rear extension takes the finished scheme to 120m². It is a heavy conversion, so you price the build at £1,700/m².

Item	Basis	Cost
Construction	120m² × £1,700	£204,000
Professional fees	10% of build	£20,400
Contingency	12% of build	£24,480
Building control + statutory	fixed estimate	£6,000
Utilities (second connection)	fixed estimate	£5,500
Subtotal (ex VAT)		£260,380
VAT on build at 5% (conversion)	5% × £204,000	£10,200
Total build budget		£270,580

Notice that the "£1,700/m²" you started with — £204,000 — is only 75% of the real number. The all-in figure you must put in your appraisal is closer to £2,255/m². Developers who appraise on the bare construction rate are systematically underpricing their projects by a quarter, and then wondering where their margin went.

Sharpen the estimate as the deal progresses

The £/m² method is for the back-of-envelope stage — deciding whether a deal is worth pursuing at all. Once you are serious, replace assumptions with evidence:

• Get two or three builder quotes against a written scope of works, not a vague description.
• For anything over roughly £200,000 of build, a quantity surveyor's elemental cost plan is money well spent and will be far more accurate than any rate card.
• Price the specification you will actually sell — buyers in your area may expect underfloor heating and a utility room, or may not care. Over-spec is wasted money; under-spec costs you on the sale.
• Lock down the groundworks and structural elements early, because that is where the nasty surprises live.

Practical takeaways

• Estimate against finished floor area, using a realistic £/m² rate for the type of work.
• Always add professional fees, contingency, statutory costs and VAT on top — the headline rate is roughly three-quarters of the real cost.
• Confirm the correct VAT rate (0%, 5% or 20%) for your specific scheme before you commit.
• Carry 10–15% contingency. The discipline of pricing it in is what separates developers who survive their first surprise from those who don't.

Building this up by hand in a spreadsheet works, but it is slow and easy to get wrong — especially the fees, contingency and VAT layers that sit on top of the build rate. Marginly's free deal appraisal calculator does it for you: enter the floor area, choose the build type, and it applies your fee and contingency assumptions automatically, then feeds the all-in build cost straight into your profit, margin-on-GDV and ROI figures. You can model three or four spec levels in a couple of minutes and see exactly where the deal needs to land before you make an offer.