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

Of all the numbers in a development appraisal, the build cost is the one most likely to sink your deal — and the one developers most often get wrong. Get your purchase price slightly off and you lose a little margin. Get your build cost off by 20% on a £300,000 scheme and you've wiped out your entire profit. With material and labour costs still elevated in 2026 after several volatile years, knowing how to build a realistic construction budget is more important than ever. Here's how experienced UK developers do it.

Start with cost per square metre

The fastest way to get a ballpark build cost is the cost-per-square-metre method. You take the gross internal floor area (GIA) of the finished scheme in m² and multiply it by an appropriate rate for the type of work. It's not precise enough to build from, but it's exactly what you need at the appraisal stage to decide whether a deal is worth pursuing.

Here are realistic 2025/26 rates for the UK (excluding London, which typically runs 20–40% higher):

Work Type	Cost per m² (GIA)
Light refurbishment (cosmetic, kitchens/bathrooms)	£500–£1,000
Heavy refurbishment (rewire, replumb, reconfigure)	£1,000–£1,800
Conversion (e.g. house to flats, barn, commercial)	£1,300–£2,200
New build (standard residential spec)	£1,600–£2,900
New build (high spec / difficult site)	£2,900–£3,800

So a 120 m² new build at £2,200/m² gives a headline build cost of £264,000. That's your starting point — not your final number.

Add the costs that sit on top of construction

The per-m² figure only covers the physical construction. A surprising number of first-time developers stop there and wonder why they run out of money. Several major cost categories sit on top:

• Professional fees — architect, structural engineer, quantity surveyor, planning consultant, party wall surveyor and building control. Budget 8–13% of the construction cost. Smaller and more complex schemes sit at the higher end.
• Site-specific costs — utility connections (a new electricity or gas connection can run £3,000–£15,000+), drainage, ground investigation, demolition, scaffolding, skips and welfare facilities. These are easy to miss from a per-m² rate.
• Section 106 / CIL — on schemes that create new dwellings, the Community Infrastructure Levy can add a meaningful sum per m². Check the rate with the local authority before you commit.
• Contingency — non-negotiable. Allow 10% for a straightforward project and 12–15% for conversions, listed buildings or anything where you can't see behind the walls yet.

Don't forget VAT

VAT treatment can swing your build budget by tens of thousands and is widely misunderstood. The headline rules in the UK:

• New build residential is zero-rated — your contractor should not charge VAT on construction labour and materials for a genuinely new dwelling.
• Conversions that change the number of dwellings (e.g. a house into flats, or a commercial building into residential) usually qualify for the reduced 5% rate rather than 20%.
• Renovating a property empty for 2+ years can also qualify for the 5% rate.
• Standard refurbishment of an existing dwelling is charged at the full 20%.

Getting the right rate applied — and using the DIY Housebuilders Scheme to reclaim VAT where eligible — can be the difference between a deal working and not. If in doubt, take advice early; the rules are specific and contractors don't always get them right.

A worked example

You're converting a redundant two-storey shop into two flats. The gross internal area is 150 m². You assess it as a conversion at £1,800/m².

Item	Amount
Construction (150 m² × £1,800)	£270,000
Professional fees (11%)	£29,700
Utility connections & site costs	£14,000
Contingency (12%)	£32,400
Subtotal	£346,100
VAT note: conversion qualifies for 5% rate	included in rates above
Total build budget	£346,100

The £270,000 headline figure became £346,100 once everything else was layered in — nearly 30% more. That is the gap that catches people out, and it's why your appraisal must use the full build budget, not just the per-m² number.

How to firm up the number before you commit

The per-m² method is for the appraisal stage. Before you exchange — and certainly before you draw down finance — you want to tighten it:

• Get a quantity surveyor's elemental estimate. For a few hundred pounds a QS will produce a far more accurate cost plan broken down by element (foundations, structure, roof, M&E, finishes).
• Get two or three fixed-price builder quotes against a clear specification. Vague specs produce vague quotes and expensive variations later.
• Pressure-test the contingency. On older buildings, assume you'll find something — damp, asbestos, inadequate foundations, dodgy wiring.
• Index for inflation if your build won't start for several months. Tender prices move, and a rate that's right today may be light by the time you're on site.

Practical takeaways

Use cost per m² to screen deals quickly, but never appraise on the headline construction figure alone. Always layer on professional fees, site costs, contingency and the correct VAT treatment. Treat contingency as a real cost, not optional padding. And firm the number up with a QS cost plan and fixed-price quotes before any money is committed. A build cost that's 20% optimistic is the single most common reason small developments lose money.

If you'd rather not wrestle with a spreadsheet every time a deal lands in your inbox, Marginly's free deal appraisal calculator builds your construction budget, applies professional fees and contingency, and flows it straight through to your profit, margin on GDV and finance costs — so you can see in minutes whether a deal stacks up before you waste a viewing on it.