How to quote for a house extension

An extension is a different animal from a single-room job. It runs for weeks or months, pulls in groundworks, structure, roof, every trade in the building, and a pile of decisions the customer hasn't made yet. Quote it as one big number and you're carrying every unknown on the build yourself. The trades who do this well stage the job, price the certain parts firmly, and handle the unknowns honestly. Here is how to structure a quote for a job this size so it protects your margin and the customer trusts it.

Why an extension isn't quoted like a small job

On a bathroom or a kitchen you can usually see most of the job before you start. On an extension you can't. There are weeks of work, ground conditions you won't know until you dig, structural requirements set by drawings and building control, and a long list of choices the customer is still making, from windows to flooring to kitchen units. Pretending you can fix one price for all of that up front is how builders end up out of pocket or in a dispute. The skill is breaking it into parts you can price firmly and parts you have to handle as allowances.

Stage the job

An extension quotes cleanly when you break it into the stages of the build. It lets the customer follow the money, it ties your payments to milestones, and it stops anything getting forgotten across a long job.

• Groundworks and foundations: setting out, digging, footings and any drainage, which depend heavily on ground conditions.
• Structure: blockwork or brickwork, steels, the floor structure and the shell going up.
• Roof: structure, covering, flashings and rainwater goods.
• First fix: making it weathertight, then plumbing, electrics and carpentry first fix.
• Plastering and finishes: boarding, plastering, screed and drying-out time.
• Second fix: kitchens, bathrooms, electrics, plumbing, doors, skirting and decoration.
• External works and making good: tying into the existing house, landscaping and a final clean.

Use provisional sums for what isn't decided

On a long job the customer hasn't chosen half the finishes yet, and you shouldn't carry that uncertainty for free. The honest way to handle it is provisional sums: an allowance for an item that's known to be needed but not yet specified, like a kitchen, bathroom suite, tiles, flooring or windows. State each one clearly as a provisional sum, explain that the final figure is confirmed once the customer chooses, and that the difference is added or credited. This keeps the headline price realistic without you guessing at a spec that doesn't exist yet, and it stops a customer assuming a basic allowance covered a premium choice.

• Label every allowance plainly as a provisional sum, not part of the fixed price.
• Say what each one assumes, so the customer can see whether their choice will land above or below it.
• Confirm each in writing once chosen, and adjust the total up or down accordingly.
• Keep them separate from your firm-priced labour so the breakdown stays honest.

Where quote and estimate sit together

An extension is the clearest case of a job that's part quote, part estimate. The measurable, drawn elements you can quote firmly. The genuine unknowns, above all the ground conditions for the foundations, you cannot. Don't fix a price as if the ground is sound and then absorb the cost when it isn't. Quote the parts you can see, treat the foundations and anything truly hidden as an estimate or a stated allowance, and agree plainly that work uncovered on site is priced and signed off before you carry on. Our guide on quote vs estimate explains exactly where that line falls.

Price your own labour and materials, and tie payments to stages

Price your labour from the realistic programme across every stage, and your materials from your suppliers and the drawings, plus the trades you're bringing in. Price your own materials and labour for the build in front of you, because two extensions of the same footprint can be wildly different jobs depending on ground, access and spec. On a job this size, don't take one big deposit and the balance at the end. Tie payments to the stages so your cash flow matches the work and the customer pays for progress they can see.

Be clear about scope, exclusions and the unknowns

Set out the scope stage by stage, list your provisional sums, and be explicit about exclusions: planning fees, building control fees, structural calculations, surveys, landscaping or anything the customer is arranging themselves. State your stage payment schedule and what triggers each one. Most of all, be honest about the unknowns up front, because a customer who understands from day one that foundations depend on the ground will take a fair extra far better than one who was promised a fixed price for everything.

For the standard sections every quote should carry, see what every professional quote should include. And because deposits and staged payments matter so much on a long build, our guide on how much deposit should a tradesperson ask for covers tying payments to milestones rather than taking one large sum up front.