A bathroom is a fortnight of known work. An extension is three months of committed labour, a supply chain, a customer living on site, and a foundation depth nobody can confirm until the digger is already in the garden. Get the foundations wrong and you’ve lost the job’s entire margin before the brickwork starts.

The way through isn’t a bigger number. It’s a quote that separates what you genuinely know from what nobody can know yet, stages the money so you’re never funding the customer’s build, and reads like a proposal rather than a figure on a compliment slip. The figures below are illustrative throughout — your suppliers, your ground and your labour decide your numbers.

Survey what the drawings don’t tell you

You’ll usually be handed planning drawings and, if you’re lucky, a structural engineer’s calculations. Neither tells you what the job costs. Walk the site with a list:

Access. Can a digger and a grab lorry get to the rear? If everything goes through a side gate by barrow, your groundworks labour changes shape entirely. Note where a skip can stand and whether it needs a permit from the council.
Existing drainage. Where do the runs go, and does the new footprint sit over one? A manhole inside the new build needs raising and sealing; a run that has to be diverted is a day plus a Building Control conversation.
Ground conditions and what’s near. Made ground, a slope, a high water table, and above all trees — a mature tree near the footprint can change the foundation design completely. This is a question for the engineer and Building Control, not a guess.
The existing structure. Condition of the wall you’re tying into, the foundation it sits on, what the roof line does, and where the existing soil pipe, boiler flue and external tap are.
Services. Gas meter, incoming water, the consumer unit’s spare capacity, and whether the existing heating system can carry more radiators or needs upgrading.
Where the customer will live. A family staying in the house through a kitchen-side extension means dust protection, temporary services and working around them. That’s real time.

Photograph all of it. In three months’ time, “the wall was already cracked” is a much easier conversation with a dated photograph attached.

Price it in stages, not as one number

Break the job into the stages you’ll actually build in. It makes your own estimating more accurate, gives the customer something legible, and gives you a natural payment structure later.

Enabling works. Site set-up, protection, temporary fencing, breaking out the existing patio or path, tree and shrub removal, disposal.
Groundworks and foundations. Dig, muck-away, concrete, and the inspections that go with them. The biggest single unknown on the job.
Superstructure. Blockwork and brickwork up to DPC and beyond, cavity insulation, lintels, beams, steel to the opening-up, floor slab or beam-and-block.
Roof. Structure, covering, flashings, fascias, guttering and any rooflights.
Windows and external doors. Usually a supplier lead time that sits on the critical path — get it in writing before you commit to a programme.
First fix. Plumbing, heating, electrics, any structural opening into the existing house.
Plaster and screed. Including drying time, which is programme, not labour.
Second fix and finishes. Sanitaryware, sockets and switches, doors and architrave, decoration, flooring, external making good and landscaping back to a sensible edge.

Price each stage from a realistic day count plus its materials and sub-trades, rather than pricing the whole thing off a rate per square metre. Square-metre rates are useful for a sanity check at the end. They are a poor way to arrive at a number.

Use provisional sums for what nobody can see yet

Firm-quoting a foundation depth from ground level is guessing with your own money. The professional answer is a provisional sum: a clearly-labelled allowance inside an otherwise firm quote, adjusted up or down against the actual cost once the work is exposed and measured. The full mechanics of pricing around what you can’t see apply here more than on any other domestic job.

On an extension, the usual candidates are foundation depth beyond the allowance, drainage diversion, unexpected ground conditions, and the condition of the existing structure you’re tying into.

An illustrative build-up only — your numbers will differ:

Enabling works & groundworks           £9,400
Foundations (prov. sum, allows 1.2m)   £4,800
Superstructure & brickwork            £11,200
Roof structure & covering              £8,600
Windows & external doors               £4,200
First fix plumbing, heating, electrics £3,100
Plastering & screed                    £4,500
Second fix & finishes                  £5,300
Drainage connection (prov. sum)        £1,900
Contingency                            £2,000
                                      -------
                                       £55,000
                            (+ VAT if registered)

State the basis next to each provisional sum — “allows for 1.2m dig; depth confirmed by Building Control on inspection, adjusted at £X per additional 300mm, agreed in writing before proceeding”. A provisional sum that only ever moves upwards stops being a provisional sum and becomes an argument.

Say plainly what isn’t in it

Extensions generate more disputes over what was assumed than over what was priced. Put the exclusions on the face of the quote, not in small print at the back. The usual ones:

Planning application and Building Control fees, and the structural engineer’s calculations
Any work required by a Party Wall agreement, and the surveyor’s fees
Asbestos survey or removal in the existing structure
Kitchen or bathroom units, worktops, appliances and sanitaryware unless separately listed
Floor coverings, blinds, curtain poles and final decoration if not specified
Upgrading existing heating, wiring or drainage beyond the new build
Reinstating garden, fencing or planting beyond a stated edge

For anything statutory — planning, Building Control, Party Wall, the applicable regulations — signpost rather than advise. Point the customer to their local authority and the guidance on gov.uk, and be clear about who is responsible for applying and paying. This is general guidance, not legal advice.

Say who supplies what, too. If the customer is buying the windows, your quote covers fitting only, and delays or wrong sizes are not on your programme or your bill. Setting that out before you start is the same discipline that prevents most scope disputes.

Stage the payments and publish the programme

You should never be more than one stage out of pocket. Set out a schedule tied to visible milestones, not to dates — a date can slip through no fault of anyone, but “foundations complete and signed off” is a fact both of you can see.

An illustrative split, adapted to the job:

Milestone	Payment
On acceptance, before ordering	10%
Groundworks and foundations complete	20%
Roof watertight	25%
First fix and plaster complete	25%
Practical completion, snagging agreed	20%

Alongside it, give an outline programme in weeks with the known lead times marked — windows, steel fabrication, a stone or composite item — and state that it assumes reasonable weather and prompt decisions. Then invoice each stage the day it’s hit, with clear terms; a big job is exactly where a slipping payment turns into a cash-flow problem.

Present it so it actually gets read

The customer is spending more with you than on anything except the house itself, and they’ll show your quote to a partner, a parent, and possibly a mortgage adviser. Give them a document that survives that: stage-by-stage costs, provisional sums flagged, inclusions and exclusions on the face of it, the payment schedule, the programme, how long the price holds, and your insurance and guarantee position.

Three competent builders will often land within a sensible range of each other. The one who wins is usually the one whose quote explained itself. If extensions and groundwork are your bread and butter, the builder’s side of quoting is where the day count and the sub-trades really decide your margin.

The takeaway

An extension is eight jobs in a queue, priced off a drawing before anyone has opened the ground. Survey the access, drainage and existing structure properly, price stage by stage from real day counts, isolate the foundations and drainage as labelled provisional sums with a stated adjustment method, list your exclusions where they can’t be missed, and tie payments to milestones you can both stand and look at. Do that and the biggest job on your books becomes the most predictable one.

Queezy is built for putting together itemised, stage-by-stage quotes on site and sending them as a branded PDF before you’ve left the drive. Download it free on iPhone.

How to Quote a House Extension (Step by Step)