How to Estimate Build Costs for a UK Property Development

Why accurate build cost estimates matter

Build costs are typically the single largest line item in any property development appraisal — and they're also the hardest to get right. Underestimate your build costs by 15% and a deal that looked like a comfortable 22% margin on GDV suddenly becomes a marginal 8% return that doesn't justify the risk. Overestimate and you'll walk away from deals that would have been profitable.

The challenge is that every project is different. A light cosmetic refurb of a two-bed flat has almost nothing in common with a ground-up new build of four townhouses — yet both need a reliable cost estimate before you can decide whether the deal works. This guide gives you a practical framework for estimating build costs at the appraisal stage, before you've appointed a QS or received contractor quotes.

Cost per square metre: the starting point

Most developers and lenders work in cost per square metre (£/m²) as the baseline unit for estimating build costs. The ranges vary widely depending on the type of work, the region, and the specification, but as of 2025/26 you can use these as a reasonable starting point for residential schemes in England:

Project Type	Typical Range (£/m²)	Notes
Light refurbishment	£500–£1,000	New kitchen, bathroom, decoration, flooring. No structural work.
Heavy refurbishment	£1,000–£1,800	Structural alterations, new electrics, plumbing, layout changes.
Conversion (e.g. house to flats)	£1,200–£2,100	Fire separation, new services to each unit, building regs compliance.
New build (standard spec)	£1,500–£2,500	Timber frame or traditional masonry, mid-range finishes.
New build (high spec)	£2,500–£3,500+	Complex designs, premium finishes, basements, difficult sites.

These figures cover the direct construction cost only — they don't include professional fees, contingency, or VAT. We'll add those shortly.

A word of caution: London and the South East typically sit at the top end of these ranges (and sometimes above), while the North of England and Wales are often 10–20% below. Always sense-check against local market conditions and recent comparable projects if you can.

What to add on top of the base build cost

The £/m² figure is your direct construction cost. To get the total build cost for your appraisal, you need to add several other items that are easy to overlook:

Professional fees (8–12% of build cost)

This covers your architect, structural engineer, quantity surveyor, and any other design consultants. For a straightforward refurbishment with minimal design input, 8% is reasonable. For a new build with a full design team, 10–12% is more realistic. On a £200,000 build, professional fees at 10% add £20,000.

Contingency (10–15% of build cost)

Never skip contingency. It's not padding — it's the realistic acknowledgment that things will go wrong. A hidden drain run, contaminated soil, an unexpected party wall dispute, or simply a subcontractor going bust mid-project. For experienced developers on straightforward refurbs, 10% is the minimum. For first-time developers, complex sites, or older buildings where surprises are likely, use 15%. On that same £200,000 build, 10% contingency adds £20,000.

Building control and warranties

Building control fees are relatively modest — typically £500–£2,000 depending on the size and complexity of the project. If you're building new-build units for sale, you'll also need a structural warranty (NHBC, Premier Guarantee, or similar), which typically costs £1,500–£3,000 per unit.

Site-specific costs

These are the items that are hardest to estimate at appraisal stage because they depend on the specific site. Common ones include: utilities connections or upgrades (£2,000–£15,000+), drainage and sewer connections (£3,000–£10,000), party wall surveyor fees (£1,500–£5,000 per neighbour if they dissent), demolition (if required), asbestos removal, and ground remediation. At appraisal stage, allow a provisional sum for these based on what you know about the site.

Worked example: converting a house into two flats

You're looking at a three-storey Victorian mid-terrace in Leeds, purchased for £180,000. The plan is to convert it into two self-contained flats. The existing floor area is 110m² and you're not adding any new space.

Item	Calculation	Amount
Base build cost	110m² × £1,500/m²	£165,000
Professional fees (10%)	£165,000 × 10%	£16,500
Contingency (10%)	£165,000 × 10%	£16,500
Building control	Lump sum	£1,200
Utilities (new gas/electric to upper flat)	Provisional sum	£4,000
Party wall fees	Two neighbours × £1,500	£3,000
Total Build Cost		£206,200

That total build cost of £206,200 is what goes into your development appraisal alongside your acquisition costs, finance costs, and selling costs. If the two flats have a combined GDV of £320,000, you can quickly see whether the deal stacks up — and in this case, with total costs (including purchase, SDLT, and finance) likely around £270,000–£280,000, you're looking at a margin on GDV of around 12–15%. Tight, but potentially workable if your numbers are solid.

Common mistakes when estimating build costs

After reviewing hundreds of deal appraisals, the same mistakes come up repeatedly. First, using outdated cost data — build costs have risen sharply since 2020. Make sure your £/m² figures reflect current market rates, not numbers from a course you attended three years ago. Second, forgetting VAT. Most refurbishment and conversion work attracts 20% VAT on labour and materials (new builds on previously undeveloped land may qualify for zero-rating). If you're not VAT-registered, this is a real cost. Third, skipping contingency to make the numbers work. If a deal only works with zero contingency, it doesn't work — full stop. Fourth, not separating labour and materials in your estimates. This matters for CIS compliance and for understanding where your money is actually going.

How to refine your estimates

The £/m² approach is a starting point for quick appraisals — it lets you assess whether a deal is worth investigating further. But before you commit to a purchase, you should refine your estimate by getting at least two or three contractor quotes based on a detailed specification. A good architect or QS can prepare a schedule of works that contractors can price against, giving you much more confidence in your numbers.

For initial appraisals, though, a tool like Marginly's free deal calculator lets you plug in your estimated build cost alongside all your other costs — acquisition, SDLT, finance, and selling costs — and instantly see your projected profit, margin on GDV, and cash required. It's the fastest way to filter out deals that don't work before you spend money on surveys and professional fees.