How to write a quote for a single-storey extension (with a real worked example)

A mate of mine priced a rear extension for a couple in Nottingham on the back of a sketch and a chat over the garden fence. Four metres by three and a half, bifolds, flat roof. He gave a single figure, they said yes, and he started digging the following month. The trench hit an old drain run that wasn't on any plan, the engineer wanted a deeper foundation by the party wall, and the steel for the opening up cost more than he'd allowed for. By the time the roof was on he'd lost about £4,800 of his own margin.

Nobody lied and nobody was daft. The quote just didn't say what was in, what was provisional and who paid for the bits that come from other people. This guide shows how to write a single-storey extension quote so those lines are in writing before the digger arrives.

Before you price anything, pin down what you're pricing

An extension quote goes wrong at the front end more than the back. Before you open a calculator, get these written down:

• The drawings and the spec. Are there approved plans and structural calculations, or is the customer expecting you to price from a sketch? If there are no drawings, quote for a survey and design stage first, or put "price subject to final drawings" at the top.
• Planning and building control. Many rear extensions go through under permitted development, but depth and height limits differ for terraced, semi-detached and detached homes and are tighter in conservation areas. Either the customer has a planning decision or lawful development certificate, or they don't. Say whose job it is. Building Regulations apply either way, so you need a Building Notice or full plans application and inspections.
• The neighbours. If you're excavating close to a shared boundary, the Party Wall etc. Act 1996 may apply. The notices and any surveyors are normally the building owner's cost, not yours. Say so on the quote.
• Access. Can a mini digger and a lorry get to the back garden, or is everything going through the house in barrows? That alone can add days.

Then walk the site. Look for manholes, gullies and downpipes in the footprint. Check the existing wall you'll be cutting into, the damp-proof course level and the ground level against the floor level. These are the things that turn into variations, and you want them on the quote as exclusions or provisional sums, not as surprises.

Split the quote the way the job is built

A quote with one figure at the bottom is an invitation to argue. A quote split into stages shows the customer you've thought it through, and it gives you somewhere to point when they ask for a change. A sensible structure for a single-storey extension looks like this:

1. Preliminaries and fees

Skips, welfare, protection to the existing house, temporary dust screens, and any fees you're paying on the customer's behalf. Show the engineer's calculations and building control charges as separate items, marked "at cost". If the customer is paying them directly, move them to the exclusions.

2. Groundworks and substructure

Strip the topsoil, dig the trenches, pour the foundations, build up to damp-proof course level, lay the floor slab with insulation and a damp-proof membrane, and connect or divert the drains. Price the foundation to the depth in the drawings and add a rate per extra metre for depth beyond that. Water, clay and tree roots are the three things that change a foundation price.

3. Superstructure

Cavity walls, lintels, the steel beam where you open up the back of the house, cavity insulation, and the temporary propping while the steel goes in. If the beam is supplied by the engineer's spec, say so. If the steel price is uncertain at quote stage, put it in as a provisional sum until you have a supplier's figure.

4. Roof

Joists, deck, insulation, covering, fascias, guttering and any rooflight. A flat roof on an extension is priced the same way as a stand-alone one, so the flat roof quote guide covers the deck, falls and weather terms in more detail. A pitched or lean-to roof needs its own breakdown.

5. Openings, first fix and finishes

Doors and windows (supply and fit), first fix electrics and plumbing, insulation, plasterboard and skim. If you're using subcontractors for the sparks and the plumber, say whether their certificates are included. Electrical work in a dwelling is notifiable under Part P, so someone has to be registered to sign it off.

A worked example: 14 m² rear extension, semi in Nottingham

Here's a quote from Adam Wójcik trading as Wójcik Building for a single-storey rear extension, 4m wide by 3.5m deep, on a 1960s semi. Cavity walls, flat roof with a rooflight, sliding doors across the back, a new opening into the kitchen. Customer is a homeowner and has planning confirmed. Wójcik Building isn't VAT-registered.

Cost build-up (materials, labour and subcontractors):

• Preliminaries: three skips, welfare, protection to the existing house = £1,350
• Structural engineer and building control fees, at cost = £1,150
• Groundworks: excavation, trench-fill foundations, drain connection allowance = £4,200
• Substructure: blockwork to DPC, 150mm slab with insulation and membrane = £3,900
• Superstructure: brick and block cavity walls, cavity insulation, lintels, steel beam and propping = £7,600
• Roof: joists, deck, warm roof insulation, EPDM, fascias, guttering, supply of rooflight = £5,100
• Openings: sliding doors and one window, supplied and fitted = £3,550
• First fix electrics and plumbing by subcontractors, certificates included = £2,600
• Internal insulation, plasterboard and skim = £2,450
• Making good where the new walls meet the existing house = £900
• Provisional sum for ground conditions and drains, adjusted to actual cost = £1,500

Add those up and you get £34,300. Check it: 1,350 + 1,150 + 4,200 + 3,900 + 7,600 + 5,100 + 3,550 + 2,600 + 2,450 + 900 + 1,500 = 34,300.

Overheads and profit at 15% on top: £34,300 x 0.15 = £5,145. That covers insurance, tools, van, admin time, the days you'll spend on the phone and the margin the business needs to stay in business. Total price: £39,445. That's about £2,818 a square metre for the 14 m² footprint, before kitchen fit, flooring and decorating.

If the firm were VAT-registered, building an extension on an existing home is standard rated, so you'd add 20% (£7,889) and the quote would need to show the VAT line. The registration threshold at the time of writing is £90,000 of taxable turnover, so a single job like this can push a small firm over it. The VAT on building work guide has the rates.

Provisional sums, exclusions and the customer's own spend

Two sections do more to protect your margin than any line in the price: provisional sums and exclusions.

Provisional sums are honest guesses for work you can't see yet. In the example above it's £1,500 for ground and drain surprises. Write next to it: "This is an allowance. Final cost will be adjusted up or down to actual cost, with photos and receipts." That sentence is what lets you charge for the old clay pipe under the trench without a row.

Exclusions tell the customer what they still have to find money for. On an extension those are usually:

• Kitchen units, worktops and fitting.
• Floor finishes, decorating and tiling.
• Radiators, underfloor heating or boiler upgrades beyond the first fix allowance.
• Planning fees, party wall notices and surveyors.
• Landscaping, patio and fencing repairs after the build.
• Anything that needs asbestos testing or removal in the existing house.

Also list the choices the customer still has to make, with dates. Late decisions on doors and finishes cause more programme delays than the weather does. Put "Doors to be ordered by [date] to hold the programme" on the quote, and add an allowance for the supplier lead time.

Payment stages that match the build

An extension is months of work and a lot of material up front. If you wait for the end to get paid, you're lending the customer £30,000. Tie payments to things you can both see:

• £4,000 on acceptance, to book the dates and order long-lead items.
• £8,000 when foundations and slab are complete.
• £9,000 when the walls are up to wall plate, with the steel in.
• £8,000 when the roof is watertight.
• £6,000 after first fix and plaster.
• £4,445 on completion, once the snag list is signed off.

That adds up to £39,445. Adjustments to the provisional sum and any agreed variations go on the stage invoice they relate to, with the photos. The stage payments guide covers how to word the milestones, and the deposit guide covers how much is reasonable up front.

If a business customer pays late, the Late Payment of Commercial Debts (Interest) Act 1998 gives you interest and fixed recovery costs, and the claim route is Money Claim Online in England and Simple Procedure in Scotland. A homeowner is a consumer, so the Act doesn't apply, but your terms can still set out what happens if a stage isn't paid. The Consumer Rights Act 2015 also means the work has to be done with reasonable care and skill, and Citizens Advice publishes plain guidance on that for homeowners.

What NOT to do

1. Don't quote from a sketch. Without drawings and an engineer's spec you're guessing at foundations and steel. Quote for the design stage first, or mark the price as subject to final drawings.
2. Don't give one lump sum. A single figure leaves you nothing to point to when they ask for a bigger window. Break it into stages so a change has a price attached.
3. Don't absorb third-party fees silently. Engineer, building control, party wall and planning costs belong on their own lines or in the exclusions.
4. Don't skip the provisional sum. Ground and drains are the biggest unknowns on any extension. If there's no allowance on the quote, the first surprise comes out of your profit.
5. Don't agree changes in the garden. If the customer wants the doors moved or the roof lifted, write it up, price it, and get a yes by email or text before you order anything. A variation order takes two minutes.

The short version

A good extension quote follows the build: preliminaries, ground, structure, roof, fit-out. It names the things you can't see as provisional, and the things other people pay for as exclusions. Add 15% for overheads and profit if that's what your business needs, then split the payments into stages. If you want to check the day rate and margin behind a number like this, the pricing guide walks through it.

Frequently asked questions

How much does a single-storey rear extension cost to build in the UK?

It varies with spec, site access and ground conditions. The worked example is a 14 m² extension at £39,445 before kitchen fit, flooring and decorating, about £2,818 a square metre. Take your own costs, add overheads and profit, and show the customer what is and is not included.

Do I need planning permission for a single-storey extension?

Many rear extensions fall under permitted development, with limits on depth, height and materials that differ for terraced, semi-detached and detached houses and are tighter in conservation areas. The customer or their designer should confirm with the council or get a lawful development certificate. Write on the quote who is responsible for it.

Does an extension need building regulations approval?

Yes. Foundations, structure, insulation, drainage, electrics and ventilation all fall under the Building Regulations, so you need a Building Notice or full plans application and inspections at each stage. Put the fees and who pays them on the quote.

Who pays for the structural engineer and party wall surveyor?

Decide before you quote and say so. Engineer's calculations are usually needed for the new opening and beams. Party wall notices and surveyors, where the Party Wall etc. Act 1996 applies, are normally the building owner's cost, so list them as an exclusion or a pass-through at cost.

How should I take payments on a job this size?

Use a modest deposit and stage payments tied to visible milestones such as foundations, walls, watertight roof, first fix and completion. The worked example uses six stages that add up to the full price, with the last one held until the snag list is done.

What if the ground is worse than expected?

Use a provisional sum or a rate per metre for extra depth of foundation and say it will be adjusted to actual cost. Photograph the trench before the concrete goes in and get the change agreed in writing the same day.