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

Why build cost estimates make or break a deal

Of all the numbers in a development appraisal, the build cost is the one most likely to sink you. Get the purchase price slightly wrong and you can usually negotiate. Get the build cost wrong by 20% on a project with a 20% margin, and your profit is gone entirely. In 2026, with material prices having settled but skilled labour still tight across much of the UK, a disciplined approach to estimating build costs is the difference between a deal that stacks and a deal that quietly loses money. This guide walks through how to build up a credible construction budget before you commit.

Start with cost per square metre

The fastest way to get a ballpark build cost is to multiply the gross internal floor area (GIA) of the finished scheme by an appropriate rate per square metre. This is how quantity surveyors produce early-stage estimates, and it is accurate enough to decide whether a deal is worth pursuing.

The rate you use depends entirely on the type of work. As a rough guide for 2026 (excluding land, fees, and VAT where reclaimable):

Work type	Typical cost per m²
Light refurbishment (cosmetic, kitchens/bathrooms)	£600–£1,100
Heavy refurbishment (rewire, replumb, reconfigure)	£1,100–£1,900
Conversion (e.g. house to flats, commercial to resi)	£1,300–£2,200
New build (standard spec)	£1,700–£2,600
New build (high spec / constrained site)	£2,600–£3,500+

So a 150m² new build at £2,100/m² gives a headline construction cost of around £315,000. London and the South East sit at the top of these ranges; the North, Midlands, and Wales typically sit 10–20% lower. Use local rates if you have them — a builder's recent quote on a comparable job beats any national average.

Add contingency — always

No development runs exactly to budget. Hidden structural issues, damp, asbestos, drainage surprises, and design changes are the norm, not the exception. A contingency is not optional padding; it is part of the real cost of the project.

For an experienced developer working on a property they know well, 10% of build cost is the minimum. For a first-timer, an older building, or a complex conversion, budget 15%. On a £315,000 build, a 12.5% contingency adds roughly £39,000. If you don't spend it, that's upside — but if you haven't allowed for it and you need it, the money has to come from somewhere, usually your margin.

Don't forget professional fees

The construction figure only covers the builder. On top of that sits a layer of professional and statutory costs that catch out developers who only budget for bricks and labour:

• Architect / designer — typically 5–10% of build cost for a full service, less for planning drawings only.
• Structural engineer — £1,500–£6,000 depending on the extent of structural work.
• Quantity surveyor — useful on larger schemes; £2,000–£8,000 or a percentage of cost.
• Planning consultant — £2,000–£10,000 if the application is non-trivial.
• Building control — £500–£2,000, via the council or an approved inspector.
• Party wall surveyor — £1,000–£3,000 per neighbour where the Party Wall Act applies.
• CDM / principal designer — required under construction regulations on most projects.

As a planning figure, allow 8–12% of build cost for professional fees on a typical conversion or new build, then refine with actual quotes.

Site-specific costs and the things people miss

Two identical buildings can have very different build costs because of what's around and underneath them. Before you fix a budget, check for:

• Utility connections — a new gas, water, or electricity connection can run £2,000–£20,000+. A new sewer connection or upgraded electrical supply is a classic budget-buster.
• Demolition and site clearance — including asbestos removal, which requires a licensed contractor.
• Ground conditions — poor ground, made-up ground, or a high water table can mean expensive foundations (piling can add tens of thousands).
• Access and scaffolding — tight urban sites cost more to work on; restricted parking and skip permits add up.
• Section 106 / CIL — the Community Infrastructure Levy is charged per m² of new floorspace in many areas and is easy to overlook.

How VAT works on development build costs

VAT treatment can swing your build cost by 20%, so it's worth understanding the basics. Most standard refurbishment and repair work is charged at the standard 20% rate. But there are valuable reliefs:

• New build dwellings are generally zero-rated — the builder charges no VAT on labour or materials for a qualifying new house or flat.
• Conversions that change the number of dwellings (e.g. a house into flats, or a commercial unit into residential) usually qualify for the reduced 5% rate on construction.
• Renovating a property empty for two or more years can also qualify for the 5% rate.

For a buy-to-sell developer, getting the VAT rate right (and instructing your contractor to invoice at the correct rate) can be worth tens of thousands. Always confirm eligibility with an accountant — the rules are specific.

A worked example

You're converting a tired three-bed house (110m²) into two flats, with a small rear extension taking the finished GIA to 130m².

Item	Calculation	Cost
Construction (conversion)	130m² × £1,650	£214,500
Contingency	12%	£25,740
Professional fees	10%	£21,450
Utility upgrade (second supply)	fixed	£6,000
Building control & warranties	fixed	£3,500
Total build budget		£271,190

Note the construction figure is eligible for the 5% reduced VAT rate as a conversion changing the number of dwellings — already reflected in the rate quoted. The headline £214,500 has become £271,190 once contingency, fees, and site costs are included — a 26% uplift on the raw build number. That gap is exactly where under-budgeted deals go wrong.

Practical takeaways

• Start with a cost-per-m² estimate, then refine with real builder quotes before exchange.
• Treat contingency (10–15%) and professional fees (8–12%) as non-negotiable line items, not extras.
• Investigate utilities, ground conditions, and CIL before you fix the budget.
• Confirm the correct VAT rate — zero-rated new builds and 5% conversions can transform the numbers.

Run the numbers before you commit

Building up a construction budget by hand in a spreadsheet is slow and easy to get wrong. Marginly's free deal appraisal calculator lets you plug in floor area, a cost-per-m² rate, contingency, and professional fees, and instantly see how the full build cost feeds through to your profit, margin on GDV, and cash required — so you can test whether a deal really stacks before you ever make an offer.