How to Price an Extension: A UK Builder's Guide

Why Getting Extension Pricing Right Matters

Extensions are bread and butter for most UK builders. They're also where the most money gets left on the table — or lost entirely. Price too high and the customer goes with someone cheaper. Price too low and you're working for nothing by the time snagging rolls around.

The difference between a profitable extension and a painful one usually comes down to how thorough your pricing is. Miss one trade, underestimate groundworks, or forget to account for BCO inspections, and your margin evaporates.

Here's a practical walkthrough of how to price a typical single-storey rear extension — 4m x 3m with a flat roof and bi-fold doors.

Step 1: Break It Down by Work Stage

The biggest mistake builders make is trying to price an extension as one lump sum. Instead, break it into stages and price each one separately. For a typical rear extension, your stages are:

Preliminaries — skip hire, scaffolding, site welfare, building control fees. These are easy to forget but they add up quickly. Two skips and scaffolding alone can be £1,200+.

Groundworks & Foundations — excavation, concrete, rebar, drainage, DPM. The cost here depends heavily on ground conditions. Standard strip foundations on decent ground might be £4,000–5,000. Piled foundations on clay can double that.

Superstructure — blocks, bricks, lintels, wall ties, mortar, steel beams. For a 4x3m extension with standard cavity wall construction, materials might run £3,500–5,000. The steel beam alone (usually a 203x133 UB for a 3m opening) is £500–800 depending on the span.

Roof — for a flat roof with EPDM membrane, you're looking at decking boards, insulation, membrane, and edge trim. Material cost for 12m² is typically £1,500–2,000. A pitched roof would add £3,000–5,000 to the total.

Windows & Doors — bi-fold doors are the big ticket item. A 3m aluminium bi-fold runs £2,500–4,000 depending on spec. Standard windows are £300–500 each.

First Fix — stud walls, door linings, electrical first fix, plumbing first fix. If you're subbing the electrics and plumbing, get firm quotes from your sparky and plumber before pricing the job.

Second Fix — plastering, skirting, architrave, doors, electrical second fix. Skim plastering at £10–14/m² for 40-50m² of wall and ceiling is £400–700.

Decoration — mist coat plus two coats emulsion. Often the customer wants to do this themselves, so clarify upfront whether it's included.

Step 2: Price Materials Accurately

Don't guess material prices. Check current prices on Screwfix, Jewson, or Travis Perkins. Prices have been volatile over the past few years — timber, steel, and concrete have all seen significant swings.

For a 4x3m extension, a realistic materials estimate in 2026 might look like:

Add 5–10% for waste and breakage. On a £16,600 materials bill, that's £800–1,660. Better to include it and not need it than to absorb it from your profit.

Step 3: Calculate Labour

Labour is where most builders have the best instinct but the worst documentation. You know roughly how long each stage takes, but if you don't write it down, it's easy to underestimate.

For a 4x3m extension with a 2-person team:

Day rates vary by region. London and the South East command £300–350/day. The Midlands and North are typically £220–260/day. Use your actual rate, not an average.

Step 4: Add Overheads, Contingency, and Profit

Overheads cover your van, tools, insurance, accountancy, phone, and other business costs. A common approach is to add 5–10% of the job value. On a £25,000 job, that's £1,250–2,500.

Contingency is your safety net for the unexpected — bad ground, hidden drainage, asbestos, or changes the customer requests mid-build. For extensions, 5–10% is standard. Less than 5% is risky. On a straightforward extension with a site survey done, 5% is usually sufficient.

Profit margin is what you actually earn after all costs. Many builders target 15–20%. Some go higher on complex work. Below 10% and you're essentially working for wages with the added risk of being self-employed. On a £25,000 net cost, 15% profit is £3,750.

Step 5: Add It All Up

This gives you a quote range of roughly £37,000–42,000 including VAT for a 4x3m single-storey rear extension with flat roof and bi-fold doors. That's consistent with the 2026 market for this type of work in most parts of England.

How BuildScope Speeds This Up

Everything above — breaking down stages, pricing materials, calculating labour, adding contingency and profit — is exactly what BuildScope automates. Describe the job, and the AI generates all of this in under two minutes. You review, adjust any numbers, and send a professional quote.

The materials prices come from real UK supplier data. The labour estimates are based on your region and trade. And the quote your customer receives shows as much or as little detail as you choose.

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