How to price an extension: a builder's method, start to finish

↑ Part of the Pricing series · How to price a job and not lose money

An extension is priced the way any job is: break it into stages, put a duration and a cost against each, add the subcontractors and the costs that are easy to forget, recover your overheads for the weeks you are on site, and put your markup on the bare cost. What makes extensions different is scale and uncertainty. The ground can surprise you, much of the work is other trades, and a few weeks of programme slip costs more than a whole bathroom. Price the known parts precisely and the unknown parts openly.

This page sits under our guide to pricing a job. It walks through a single-storey extension in the order you would price it. The figures in the example are an illustration built from one set of costs, not a market rate: use your own.

Part 1Read everything before you price anything

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Most extension losses are written into the quote on day one, by pricing from a sketch. Before you start:

• The drawings and the structural engineer's details. Steel sizes, padstones, foundation depths and any special details change both the materials and the days.
• Building control requirements, and who is paying for the inspections. If the customer has not sorted building control, say on the quote who will.
• The ground. Trial holes if you can, or at least an honest look at the soil, nearby trees and drains. Foundations are the part most likely to move.
• Access. Can a digger, a skip and the deliveries get to the back of the house? Hand-balling muck through a terrace adds days you will not get back.
• What is excluded. Kitchen, flooring, decoration, the party wall surveyor, planning fees. Write down what is not in your price as clearly as what is.

Part 2Break it into stages

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Work through the job in the order you will build it, and put a duration on each stage:

• Groundworks: set-out, excavation, foundations, drainage, slab.
• Shell: walls to plate, steels and lintels, roof structure, roof covering.
• Openings: windows and doors, the opening into the existing house.
• First fix: electrics, plumbing, heating.
• Finishes: plastering, second fix, making good to the existing house.

Each stage has your own labour, your materials, and the trades you bring in. Pricing stage by stage means a mistake in one stage does not hide in the total, and it gives you natural points for stage payments.

Part 3Your labour, the subcontractors and the materials

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Your labour is the days you and anyone you employ will spend, at what each day costs you. Use your real wage figure here, not your selling day rate, because overhead and markup are added separately below. Our guide to working out your day rate explains the difference.

Subcontractors are a cost like any other: the electrician, the plasterer, the roofer, the groundworker if you use one. Get their prices in writing before you send yours, and make sure their scope matches yours, so a gap between the two is not yours to fill.

Materials come off a proper take-off from the drawings, not a guess per square metre. Ask the merchant for a quote on the bulk items and allow for waste on everything.

Part 4The costs that are easy to forget

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Extensions carry more hidden costs than most jobs: skips and muck-away, scaffold, plant hire, welfare and site security, temporary weather protection when the opening is made, the protection of the customer's existing floors, and the making good where new meets old. Each is its own line, costed like the work.

Part 5Allow for what nobody can see yet

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Some of an extension cannot be priced until the ground is open. Put those parts on the quote as provisional sums, with the basis they will be charged on, rather than folding a guess into the price:

• Deeper or different foundations than the drawings assume.
• Drainage that turns out to be in the way, or in poor condition.
• Anything building control asks for that is not on the drawings.

Items the customer has not chosen yet, such as the roof lights or the bifold doors, go in as PC sums.

Part 6The worked example

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A single-storey rear extension, priced on the method above. Your own labour is costed at £250 a day, overhead recovered at £55 for each working day on site, and the markup is 20% of the bare cost.

Your labour, 30 days at £250	£7,500.00
Second operative, 25 days at £180	£4,500.00
Materials from the take-off	£14,800.00
Subcontractors: electrician, plasterer, roofer	£6,200.00
Skips, muck-away, scaffold and plant	£2,600.00
Bare cost of the job	£35,600.00
Overhead recovery, 30 days at £55	£1,650.00
Markup at 20% of the bare cost	£7,120.00
Price before VAT and provisional sums	£44,370.00

The provisional sums for foundations and drainage sit on the quote beside that figure, labelled, with how each will be charged. The overhead is never shown to the customer as a line: it is spread through the line prices. And a 20% markup is a margin of about 16% on this job, which is the number to compare with what you need. Our guide to markup vs margin explains why the two differ.

If you are VAT registered, VAT goes on top, and a quote to a homeowner shows the total including it.

Part 7The programme is part of the price

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On a job this size, time is money in two ways. Every extra week on site is a week of your labour and your overhead that the price has to cover, so be honest about the programme and price the weeks it will really take. And every week you are owed money is a week you are funding the customer's build, so put the stages on the quote and invoice each one when it is reached.

Common questionsPricing an extension: quick FAQ

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How do I price an extension?

Read the drawings, the engineer's details and building control requirements first. Break the job into stages, cost your labour, materials and subcontractors for each, add skips, scaffold and other forgotten costs, recover your overheads for the days on site, add your markup to the bare cost, and put the parts you cannot see as provisional sums.

Should I price an extension per square metre?

Not for the quote. A per-square-metre figure is a rough check at best, because foundations, access and finishes vary too much between jobs. Price it from a take-off and a programme.

How do I deal with foundations I cannot see?

Price the foundations the drawings show, and add a provisional sum for anything deeper or different, with the basis it will be charged on. That keeps the rest of the price firm.

Should the quote include the kitchen and decoration?

Only if you are doing them. Either way, say so on the quote. Most disagreements on extensions are about what was and was not included.

What payment stages suit an extension?

A deposit, then stages at visible points such as foundations complete, shell watertight, first fix done, and a final balance on completion. Put them on the quote before you start.

This guide is general information for UK tradespeople, current to the best of our knowledge in 2026, and is not tax, accounting or legal advice. Rates, thresholds and how they apply to you change, so confirm your position with HMRC, a qualified accountant or a solicitor before relying on it. TradeDraft accepts no liability for decisions made on the basis of this guide. Questions: hello@tradedraft.co.uk.