How to price a house extension in the UK, step by step

Pricing an extension is a process, not a talent. Builders who win work at a healthy margin follow the same sequence every time: understand the job, measure it, build the cost up in layers, then add the money that keeps the business alive. Here is that sequence, in the order a quantity surveyor would run it.

Step one: pin down what you are actually building

Most bad prices die at this stage, before a single number is written. A "single storey rear extension" can mean a simple flat roof box on strip footings, or a vaulted ceiling with structural glazing, underfloor heating and a full knock-through with steels. Same phrase, wildly different money.

So before you price anything, get the scope nailed down. Read the drawings if there are any, walk the site, and write a short scope list in plain English: size, roof type, openings, structural work, finish level, and what the client is supplying themselves. If the client cannot answer a question, write down the assumption you are making. That list becomes the backbone of your quote and your defence when the job changes later.

Step two: measure the job, do not guess it

Quantities are the difference between an estimate and a wish. For a typical extension you want, as a minimum: footprint area, wall areas (external skin, internal skin, internal partitions), roof area, floor area, the number and size of openings, drainage runs, and the big one-off items such as steels, bifolds and the kitchen if it is in scope.

Measure from the drawings where you have them, and check the critical dimensions on site. Where drawings are thin, measure the existing property and sketch the new footprint yourself. If you want a deeper method for this, see our guide to reading architects' drawings before you quote.

Once you have quantities, every price becomes checkable. Forty square metres of cavity wall at your blockwork rate is a fact you can stand behind. "About fifteen grand for the shell" is not.

Step three: build the cost up in layers

Work through the job in build order and price each element from its quantities: substructure, shell, roof, external joinery, first fix, plaster, second fix, decoration, external works. For each line you are combining labour time, material cost with a waste allowance, and any plant or skips that element needs.

This layered build-up is exactly what a bill of quantities gives you, and it is worth the effort even on smaller jobs. It shows the client you have priced their job rather than plucked a figure, and it gives you an itemised position to negotiate from when they ask what can come out to hit budget.

Step four: the layers most builders skip

Two layers separate professional pricing from hopeful pricing, and they are the two most often missing from domestic quotes.

Preliminaries are the costs of running the site rather than building the thing: scaffold, welfare, skips, temporary protection, small tools, parking, and the hours you spend managing rather than laying blocks. On a typical extension these commonly land somewhere around 8 to 15 percent of the job. Left out, they come straight off your profit. We cover the full list in preliminaries explained for builders.

Contingency is priced risk. Openings up an old wall, ground you have not seen, a client who has not chosen tiles yet: these have a cost, and many firms allow 5 to 10 percent depending on how much of the job is unknown. How to size it properly is a topic of its own, covered in our contingency guide.

Step five: overheads and profit go on last, and on purpose

Your overheads, the van, the insurance, the accountant, the phone that never stops, exist whether or not this job goes ahead. They have to be recovered across the year's work, so work out your annual overhead figure, spread it across your expected productive weeks, and load it into every quote deliberately.

Then add profit on top, as a separate decision. Profit is not what is left over; it is what you charge for carrying the risk and running the business. Many small firms target somewhere in the region of 15 to 25 percent combined overheads and profit on domestic work, adjusted for how busy they are and how risky the job looks.

Step six: sanity check the number, then present it well

Before the quote goes anywhere, cross-check it three ways: against your build-up (do the quantities and rates hold?), against a square metre benchmark for your region and spec level, and against your gut, because a number that feels wrong usually has a reason. If the three disagree, find out why before the client does.

Then present the quote as a proper document: scoped, itemised, with exclusions and assumptions stated. Presentation is a price signal, and a clear breakdown wins against a cheaper single figure more often than you would think.

The honest catch in all of this is time. Done by hand, this process takes an evening or two per quote, which is exactly why so many builders shortcut it. That is the gap tools like QS Quoter exist to close: it builds the full measured bill of quantities from your job description or drawings at your own rates, with every line editable, so the method above uses a structured workflow instead of two evenings. You still make the decisions; the arithmetic and the document stop eating your week.