Pricing and quoting, 29 October 2026
How to price an extension or loft as a small builder
Price an extension or loft conversion from the bottom up, stage by stage: measure the quantities from the drawings, cost the materials at today's merchant prices, count the labour days for each stage, add subcontractors, preliminaries such as scaffold and skips, your overheads per day and a contingency, then your margin. Don't price from a cost per square metre. Use that only to sense-check the total.
What do you need before you can price it?
A set of drawings, a structural engineer's calculations if beams or new foundations are involved, and a clear answer on planning and building regulations. Without those, any figure you give is an estimate, and you should call it one.
How do you break the job into stages?
Price the job in the order you'll build it. Each stage gets its own materials, labour days and subbies, which makes the price easier to check and turns straight into your stage payments.
For a rear extension, the stages are usually: setting out and groundworks, foundations and drainage, walls to wall-plate, roof structure and covering, windows and doors, first fix, plastering, second fix, floors and finishes, and making good to the existing house. For a dormer loft: scaffold and strip-out, steels and floor joists, dormer structure and roof, windows, stairs, first fix, insulation and plasterboard, fire protection, second fix and finishes.
The take-off is the slow part: lengths of timber, square metres of board and insulation, numbers of blocks. A renovation and extension builder we work with, four or five staff doing high-end domestic work, compared a roof timber take-off made from the drawings against his QS's figures and found it "pretty much bang on". Whoever does it, the take-off has to come from the drawings, not from memory of the last job. Quantity take-off shows the method.
Worked example: pricing a dormer loft conversion
Here is a rear dormer loft conversion on a semi-detached house, with one bedroom and a shower room, priced the way a two-person builder with subbies would build it. Every figure is an assumption for this example. Use your own.
Rear dormer loft conversion, priced stage by stage
A two-person team at a true cost of £262 a person per day including overheads, 30 working days on site.
| Own labour2 people × 30 days × £262 | £15,720 |
| Materials: timber, steels, insulation, board, windows, stairsfrom take-off, with 10% waste | £14,500 |
| Subcontractors: electrician, plumber, plasterer, rooferfrom their quotes | £9,800 |
| Scaffold for the durationhire quote, 8 weeks | £2,400 |
| Skips and waste4 skips | £1,200 |
| Structural engineer and building control feesif in your price | £1,600 |
| Total cost | £45,220 |
| Contingency for what's found when you open up5% of cost | £2,261 |
| Cost including contingency | £47,481 |
| Price at a 15% margin, excluding VAT£47,481 ÷ 0.85 | £55,860 |
| VAT at 20%standard-rated: work to an existing house | £11,172 |
| Price to the homeowner | £67,032 |
Two things decide whether this job makes money: the 30 days and the contingency. If the team takes 35 days, the extra 10 man-days cost £2,620, which eats all the contingency and some of the profit. That's why big domestic jobs often have the thinnest margins, and why your labour days need to come from jobs you've timed.
What are preliminaries and why do builders miss them?
Preliminaries are the costs of running the site rather than building the thing: scaffold, skips, welfare, temporary protection, waste, parking, plant hire, site set-up and your time managing it. They're easy to forget because none of them shows on the drawing. On a loft, scaffold alone can run for the whole job; price it for the real duration plus a margin of error, and say in the quote what happens if the programme slips for reasons outside your control.
Should you charge a project management fee?
If you're running the job, coordinating subbies and dealing with building control, that's work, and your price should pay for it. Some builders carry it inside their overheads; others show a separate fee as a percentage of the build cost; the renovation builder above discussed 10% with us. Whichever you choose, make sure the time is in the price somewhere. Project management fee for an extension covers how to set and present it.
How do you present the price to the client?
Give a single fixed total with a stage breakdown, clear exclusions, provisional sums for anything you can't price yet (such as a kitchen the client hasn't chosen), and a payment schedule tied to stages. Don't send every option as one long list: a builder we spoke to in the North East sent a multi-option quote and the client "nearly had a heart attack" at the combined total. Price the core job, then show options as separate add-ons. See quotes with options.
Even a single loft conversion can come with a long tender document and several attachments. Read the whole pack before you price, and list every assumption you make in the quote, because the documents are the contract. Reading a tender pack quickly helps.
Where Nexley fits
Nexley is the AI back office for UK trade firms: five AI assistants run your tenders, quotes, buying, scheduling and invoicing. It connects to the tools you already work in, you run it all from WhatsApp, and you approve everything. £799 a month.
For extensions and lofts, Nexley takes off quantities from the drawings, prices materials from your merchant invoices, applies your day rates, prelims and margin, and drafts a staged quote in your branding. You send the drawings and a voice note on WhatsApp, check the draft, and approve it before it goes to the client.
Common questions
How much does a loft conversion cost per square metre?
It depends on the type of loft, the region, the spec and access, so a single figure misleads more than it helps. Price yours from quantities and labour days, then compare the total per m² with your own past jobs as a sense check.
Do I charge VAT on an extension?
Work on an existing house, including extensions and loft conversions, is normally standard-rated at 20% if you're VAT-registered. HMRC has exceptions, such as an extension that creates a separate new dwelling, so check VAT Notice 708 or ask your accountant on anything unusual.
How much contingency should I add?
Enough to cover what you can't see until you open up: rotten timbers, poor foundations, hidden services. The example uses 5% of cost; set yours from what you've found on past jobs, and go higher on older properties. Explain on the quote what the contingency covers.
Should I price from the architect's drawings or my own survey?
Both. Price from the drawings, then check them against the house. Note any differences as assumptions in your quote.
Sources
- Planning Portal: Loft conversion planning permission
- Planning Portal: Is building regulations approval needed for a loft conversion?
- Planning Portal: Loft conversion fire safety
- London Borough of Richmond upon Thames: Serving a notice under the Party Wall Act
- GOV.UK: Buildings and construction (VAT Notice 708)
Every rule, rate and date above was checked against these sources on the date shown at the top.
Topic: Pricing and quoting
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