How to price a single-storey extension in 2026: a builder's worked example

Extensions are the bread and butter of domestic building, and they are also where the most money is lost. Not on the big items, which everyone prices, but on the forty small ones that get forgotten and the prelims that get squeezed. This article walks through a realistic build-up for a straightforward single-storey rear extension so you can check your own method against it.

The example is a 4 metre by 6 metre flat-roofed rear extension on a 1930s semi: cavity walls, strip foundations, a warm flat roof, bi-fold doors, two rooflights, the kitchen moving into the new space. Figures are illustrative; your own rates, your region and the specification will change them. The point is the structure.

Step 1: measure it properly

Before pricing anything, take off the quantities: foundation length and depth, below-ground blockwork, cavity wall area less openings, roof area, floor area, lengths of drainage, number of sockets and lights, metres of skirting and plaster. For a job this size it takes an hour by hand. If you have the architect's PDFs, EstiMate's AI take-off does it in minutes and shows you which sheet each quantity came from. The free concrete and brickwork calculators are handy for cross-checking.

Step 2: build up each element

Groundworks and foundations

Break out and remove the existing patio and paving. Excavate 20 metres of trench to a depth that depends on your ground and trees (budget for 1 metre and check with building control). Concrete the foundations. Lay below-ground blockwork and the floor: hardcore, sand blinding, damp-proof membrane, insulation, concrete slab or beam-and-block. Allow for drainage: a new manhole, connection to the existing run, and the gully for the new rainwater. Skips: at least four at this stage.

Where it goes wrong: unexpected depth of foundations, a sewer in the way requiring a build-over agreement, and tree roots. Put a stated allowance in the quote and a rate per extra cubic metre.

Structure

Cavity walls in block and facing brick to match, with insulation, wall ties, cavity trays and lintels over the openings. The structural opening between the house and the extension needs a steel beam, padstones and temporary works. Price the steel from the engineer's drawing, including the delivery and crane or genie lift if access is tight.

Roof

Flat roof: firrings and joists, decking, vapour control layer, insulation, single-ply or GRP membrane, fascias, guttering, and the two rooflights with their upstands. Insulation thickness is dictated by Part L, and the upstand heights by the rooflight maker.

Openings and glazing

The bi-fold or sliding doors are often the single most expensive item after labour. Get a written quote with the survey, the threshold detail and the lead time, and state the supply price in the quote with the client's chosen specification.

Services

Electrics: a new circuit or two, sockets, lighting, the cooker supply, and the certificate. Plumbing: hot and cold to the new kitchen position, the dishwasher and washing machine, the radiator or underfloor heating, and the boiler's capacity check. Make clear what the kitchen fitter does and what you do.

Finishes

Plasterboard and skim, or render-and-set on blockwork. Skirtings and architraves. Flooring preparation (the kitchen floor finish is usually a client item). Decoration: mist coat and two coats. External: the brick cleaning, pointing to match, and making good the garden.

Step 3: the items everybody forgets

• Building control fees and the structural engineer
• Party wall notices on a semi, and the surveyor if the neighbour dissents
• Scaffolding, even on a single storey, for the roof and the gable
• Protecting the house: floor protection, dust screens, the cleaning at the end
• Temporary kitchen for the client, or at least a clear statement that it is their problem
• Asbestos: artex ceilings and old soffits in a 1930s house
• Disposal of the old kitchen, doors and patio
• Making good where the old back wall is removed: floor levels, the ceiling junction, the old external wall now internal
• Welfare and the site toilet

Step 4: prelims and margin

Prelims on a domestic extension typically come to 8% to 15% of the works: supervision, scaffolding, skips, welfare, insurance, small plant, protection and cleaning. Price them as a list, not a percentage, so you can see them. Then add your overhead recovery and profit. A builder with an office, a van, insurance and a pension needs 20% to 30% on top of direct costs to stay in business; the ones who add 10% are the ones who disappear in year three.

Step 5: present it

The client sees a priced summary by element, a list of inclusions and exclusions, provisional sums for anything unknown, a programme, and the payment stages. That one document, done well, wins the job more often than a lower number. EstiMate turns the build-up into a branded PDF quote with optional extras the client can tick, a contract they can sign online and payment milestones that invoice themselves. Try it free on your next extension.