How to Estimate Build Costs for a UK Property Development

Why your build cost estimate makes or breaks the deal

Of every line in a development appraisal, the build cost is the largest movable number. SDLT is fixed by HMRC. Finance is priced by the market. Purchase price is set at exchange. But build cost is an estimate — and the gap between a good estimate and a bad one is often what separates a profitable scheme from a painful one. Get it 10% wrong on a £300,000 build, and you've burned £30,000 of margin before a single brick has moved.

This guide covers the four components of a credible build cost estimate: the £/m² rate, contingency, professional fees, and VAT. Get all four right and your appraisal will hold up to scrutiny from a lender, a monitoring surveyor, and — most importantly — reality.

1. Start with £/m² for your project type

The fastest way to sanity-check a build cost is to apply a rate per square metre of Gross Internal Area (GIA). The right rate depends heavily on what you're doing. Rough 2025/26 ranges for England (outside London — add 20–35% inside the M25):

Project Type	Typical Range (£/m² GIA)
Light refurbishment (cosmetic + kitchens/bathrooms)	£500–£1,000
Heavy refurbishment (rewire, replumb, replaster)	£1,000–£1,800
Conversion (HMO, flats from house, garage to dwelling)	£1,200–£2,100
New build (standard residential)	£1,800–£2,800
Loft conversion (standalone)	£1,500–£2,400
Single-storey rear extension	£2,000–£3,000

These are build cost only — they exclude professional fees, finance, contingency, and VAT. Treat the bottom of each range as best-case and the top as realistic for most jobs once specification, access, and groundworks are factored in.

Don't apply a single rate across mixed work. If you're extending and also refurbishing the original house, calculate each element separately — the extension might be £2,500/m² while the existing fabric refurb is £900/m². A blended average hides the truth.

2. Contingency — the most under-budgeted line on an appraisal

Contingency is the buffer for the things you haven't seen yet: the rotten joists behind the plaster, the asbestos in the airing cupboard, the drain run that doesn't match the searches, the building control officer who wants additional fire separation. On almost every project, contingency gets spent.

Sensible benchmarks:

• New build on a clean site: 7.5–10% of build cost
• Standard refurbishment: 10–12.5%
• Heavy conversion (HMO, flats from house, period property): 12.5–17.5%
• First-time developer or unfamiliar build type: add 2.5–5% on top of the above

If your appraisal only works at 5% contingency, the deal doesn't really work. A lender will not accept it, and the first invoice for unexpected groundworks will wipe out your margin.

3. Professional fees — budget 8–14% of build cost

Professionals are not optional, and their fees add up faster than most developers expect. On a typical residential conversion, you should expect to engage:

• Architect — 4–8% of build cost for full RIBA stages 1–4; less if you stop at planning
• Structural engineer — £1,500–£5,000 depending on complexity; more if you're forming new openings or adding storeys
• Planning consultant — £1,500–£6,000 if you have material planning issues, change of use, or are challenging policy
• Building control — £500–£2,500 (either local authority or an Approved Inspector)
• Party Wall surveyor — £900–£1,800 per adjoining owner if disputes arise; can quickly become the biggest fee on a tight site
• Quantity surveyor — optional but invaluable on builds over £400,000; expect 1–2% of build cost
• SAP / EPC / air tightness testing — £400–£1,500 combined for new build or major works

On a typical £300,000 conversion, professional fees often land at £25,000–£40,000. If your appraisal has only £8,000 in the professional fees line, you've left a hole.

4. VAT — the line developers most often get wrong

VAT on UK construction works is one of the most confusing areas of property development, and getting it wrong on a £250,000 build means a five-figure surprise. The headline rules:

• Standard rate (20%) applies to most refurbishment, repair, and extension work to existing residential dwellings. You generally cannot reclaim this if you're selling the finished property as a residential investor.
• Reduced rate (5%) applies to converting a non-residential building into a dwelling, converting a single dwelling into multiple dwellings (or vice versa), and to renovations of properties that have been empty for at least two years. Your contractor must charge 5% if the work qualifies — and many don't realise this.
• Zero rate (0%) applies to the construction of new dwellings and the first grant of a major interest in a new dwelling. Build a new house from scratch and sell it: the contractor zero-rates their invoices.

The most common mistake is assuming all conversions attract 5% when they don't, or assuming refurb work is recoverable when it isn't. If you're doing a qualifying conversion, brief your contractor and their accountant before they invoice — once 20% has been paid in error, getting it back is slow and uncertain.

Worked example: 180m² Victorian terrace converted to two flats

Line	Calculation	Amount
Build cost	180 m² × £1,600 (heavy conversion)	£288,000
Contingency	12.5% of build	£36,000
Professional fees	10% of build	£28,800
VAT on build	5% reduced rate (qualifying conversion)	£14,400
Total construction cost		£367,200

Compare that to a developer who pencilled in £288,000 flat and called it done. The gap — almost £80,000 — is the difference between a confident appraisal and an optimistic one. And had they assumed 20% VAT rather than the 5% reduced rate this conversion qualifies for, they'd have over-budgeted by £43,000 the other way and walked from a viable deal.

Practical takeaways

• Never use a single £/m² rate across mixed work — break the build into components
• Set contingency at 10% as a minimum, 12.5–15% on conversions, 15%+ on period property or first-time builds
• Budget 8–14% of build cost for professional fees and list every consultant individually
• Confirm the VAT rate that applies before signing the contractor — 5% on qualifying conversions is worth claiming
• If the deal only works with optimistic numbers in any of these four lines, it doesn't work

Running the numbers properly

A spreadsheet works, but it's slow and the assumptions are easy to lose track of across multiple deals. Marginly's free deal appraisal calculator handles all four components automatically — £/m² benchmarks by project type, contingency, professional fees as a configurable percentage, and the correct VAT rate for your conversion — so you can stress-test a deal in minutes and see where the build cost has to land for the numbers to stack.