How to Price a Single-Storey Extension
A single-storey extension is the bread-and-butter big job, and the one most often underpriced. Cost it stage by stage from the ground up, not on a rate per square metre.
A single-storey rear or side extension is the job that keeps a lot of building firms busy, but it is also where a loose quote quietly eats a season's profit. The unknowns are underground and behind the plaster, and a blanket square-metre rate ignores exactly the things — foundations, drainage, the state of the existing wall — that decide whether the job makes money.
This guide breaks an extension into the stages that actually drive the cost, from groundworks to fit-out, so you can build a quote that covers your labour, your sub-trades, building control and a proper contingency — and stands up when the customer moves the bifolds or upgrades the kitchen halfway through.
Key takeaways
- Price the stages separately — groundworks, shell, weathertight, first fix, plaster and fit-out — not a flat rate per square metre.
- Groundworks and drainage are the biggest unknown; the ground you can't see decides the foundation cost.
- Building control fees, a structural engineer's design for any openings, and inspections all belong in the price.
- Use PC sums for kitchens, glazing and finishes so a customer upgrade is charged as a variation, not absorbed.
- It is a weeks-long job — invoice in stages tied to milestones and keep a contingency for what the ground hides.
Break the extension into stages
An extension runs as a sequence, and pricing each stage separately is what makes the total defensible. The stages are: set-up and groundworks (dig, foundations, drainage, oversite and slab); the shell (walls up to plate, any steel over openings, roof structure); weathertight (roof covering, windows, external doors); first fix (electrics, plumbing, any heating); plaster and screed; then second fix and fit-out.
Costing stage by stage lets you estimate labour honestly and see where a particular extension is heavier than the last — a sloping site, a deep dig, a wide opening into the existing house. A single square-metre rate averages all of that away and is how builders get caught out on the jobs that are genuinely harder.
Groundworks and drainage — the biggest unknown
Groundworks are where the real risk sits, because the cost is decided by ground you cannot see until the digger is in it. Foundation depth depends on the soil, trees and any drains, and building control can require you to go far deeper than you priced. Existing drainage crossing the footprint may need diverting, and connecting the new drainage correctly to the existing system is a Part H requirement that inspectors check before it is covered.
Price the groundworks on a considered assumption — a stated foundation depth and drainage scope — and state that assumption clearly on the quote, so that if the dig reveals soft ground, a high water table or an unexpected drain, the extra depth or diversion is a priced variation rather than a hit to your margin.
State your groundworks assumptions
Put your assumed foundation depth and drainage scope on the quote in writing. When the ground turns out deeper or wetter than assumed, the extra is a variation you agreed the basis for up front — not an argument.
Doing this on a job? Signed variations — part of TradePlanr. Free to start, no card needed.
A worked stage breakdown
It helps to see how the money splits across a typical single-storey extension. The proportions below are indicative — every job differs with size, spec and site — but they show where the cost sits and why the early, hidden stages deserve the most care in pricing.
| Stage | What it covers | Typical share of build cost |
|---|---|---|
| Groundworks & drainage | Dig, foundations, drainage, oversite and slab | 20–30% |
| Shell | Walls, steels over openings, roof structure | 20–25% |
| Weathertight | Roof covering, windows, external doors | 10–15% |
| First fix | Electrics, plumbing, heating carcass | 10–15% |
| Plaster & screed | Plastering, floor screed, prep | 8–12% |
| Second fix & fit-out | Kitchen, finishes, decoration, snagging | 15–25% |
Build the quote from real line items
TradePlanr lets you build an extension quote from saved lines for each stage at your own rates, so every estimate starts from real numbers and converts straight to staged invoices as the job progresses.
Sub-trades, building control and compliance
An extension pulls in a groundworker, a bricklayer, a roofer, an electrician, a plumber and a plasterer. Price each sub-trade as its own line at the rate it charges you — never bury a sub-trade inside your own hours, or you pay their invoice out of your margin. Keeping them visible also makes a later change easy to price as a variation.
The work is fully notifiable and goes through building control, and any structural opening into the existing house needs a structural engineer's design for the beam. Cost the engineer, the building control application and inspection fees, and any electrical certification under Part P into the quote from the start — they are part of delivering a compliant job, not free extras.
Contingency, variations and staged invoicing
Add a contingency for what the job hides — soft ground, a rotten wall plate on the existing structure, a level that is nowhere near where the drawing assumed. On a job with this much underground and behind-the-wall risk, a sensible contingency is prudence, not padding, and it is far easier to explain up front than to claw back later.
An extension runs for weeks, so the money must come in as you go. Invoice in stages tied to milestones — groundworks complete, shell up, weathertight, plastered, completion — with card payment links, and capture every change as a signed variation, because on a job this long the customer will change their mind about the glazing, the kitchen or the finishes.
Stage the money, sign the changes
TradePlanr lets you invoice an extension in stages with card payment links and capture signed variations on site, so cash flow keeps pace with the work and an upgraded kitchen or wider opening never turns into an invoice dispute.
From guidance to action
Frequently asked questions
Why not just price an extension per square metre?
Because a flat square-metre rate averages away exactly what drives the cost — foundation depth, drainage, the state of the existing wall and the size of the opening into the house. Pricing stage by stage from the ground up captures those variables and protects your margin on the harder jobs.
What is the biggest cost risk on an extension?
Groundworks. Foundation depth and drainage are decided by ground you cannot see until you dig, and building control may require you to go deeper than assumed. State your foundation and drainage assumptions on the quote so any extra depth or diversion is a priced variation.
Does a single-storey extension need building control?
Yes. The construction work is notifiable and goes through building control by full plans or building notice, with inspections at key stages, plus a structural engineer's design for any opening into the existing house. Build those fees and your time managing inspections into the price.
How should I invoice a job this size?
In stages tied to milestones — groundworks complete, shell up, weathertight, plastered, completion — rather than all at the end. Staged invoicing with a materials deposit keeps cash flow in line with the work over the weeks the extension takes.
Related guides
Building Regulations for Extensions
Which Approved Documents apply, building control routes, and sign-off on a domestic extension.
How to Price a Kitchen Fit-Out
Break a kitchen fit-out into stages and trades, then build a quote that holds its margin.
CDM 2015 for Small Builders
Your CDM duties on small and domestic jobs, and the construction phase plan you need.
