Estimating

How to price a house extension: a builder’s method

5 October 2026 13 min read

The short answer

Price a house extension from a fixed scope, not a conversation. Get the Building Regulations drawings, the structural calculations and a specification, visit the site, then build the cost stage by stage: preliminaries, demolition, substructure and drainage, superstructure and roof, the opening and steels into the existing house, first fix, plastering, second fix, finishes, making good and external works. Price labour at its true cost, materials from a measured take-off, and plant, waste and subcontract quotes separately. Add the costs that sit outside the build, such as building control charges, the structural engineer and party wall surveyors, then provisional sums for what cannot be seen, then overhead, then margin. In England an extension normally needs Building Regulations approval, and it is standard-rated for VAT. Write the exclusions down.

At a glance

Building Regulations: Approval needed for most extensions
Full plans decision: Within 5 weeks, or 2 months with consent
Building notice: Smaller projects; work can start 2 days after
Party structure notice: At least 2 months before work (s.3)
Excavation near a neighbour: At least 1 month’s notice (s.6)
VAT on an extension: Standard-rated, 20%
Domestic client duties: Pass to the contractor (CDM reg 7)
Order of the build-up: Cost, then overhead, then margin

What do you need before you can price an extension?

A fixed scope. An extension priced from a chat and a sketch is a guess with a total at the bottom, and the guess is usually low, because the parts nobody drew are the parts nobody priced. Before you put a figure on it, get three things.

Drawings at the right stage. Planning drawings show what the extension looks like. Building Regulations drawings show how it is built: foundation depth, wall build-up, insulation, roof construction, drainage runs and the opening into the house. Price from the second set, and note the revision you priced from.
Structural calculations. The steel over the new opening, the padstones it bears on, the lintels and the foundation design. Without them the steel is a guess, and the steel and its temporary support are where an extension price most often goes wrong.
A specification. Roof covering, facing brick, windows and doors, floor build-up, heating, electrical layout and finishes. Where the client has not chosen yet, a stated allowance goes in its place.

Then settle who does what. Is the client buying the kitchen or the bifolds? Who applies for building control? Each answer moves cost into or out of your price.

No Building Regulations drawings, no calculations and no specification means you are giving an estimate built on assumptions. Write the assumptions down with it.

Whether your figure binds you depends on how it is worded. See is a builder’s quote legally binding?

What should you check on the site visit?

Walk the site with the drawings in your hand and look for what the drawings cannot show.

Access. The side passage, whether a skip fits on the drive, where a mini digger gets in and where materials go. Barrowing everything through the house is labour and protection.
Ground. Slope, made ground, clay, standing water, and whatever is under the footprint.
Drains. Lift the covers for the runs and depths. A manhole inside the footprint changes the drainage design, and a sewer on the sewer map brings in requirement H4 of the Building Regulations.
Services. Meters, the boiler flue, the soil stack, the outside tap. Anything on the rear wall the extension covers has to move, and somebody has to price moving it.
Trees. Near the footprint, the neighbour’s included. On shrinkable clay they can take foundations deeper than drawn.
The boundary. A neighbour’s wall or foundation within a few metres is a party wall question before it is a pricing one.
The existing house. Wall construction, DPC and floor levels, and its age. Opening up an older house can disturb asbestos: read which asbestos survey you need before pricing the demolition.
The family. An occupied house means dust screens, a working toilet and a tidy-up every evening. All of it is time.

Which approvals and notices does an extension need?

The rules below are for England. Most approvals are the client’s or the designer’s to obtain, but they set your programme and some of them land in your price.

Planning permission or permitted development

Many single-storey extensions are permitted development and need no planning application. The Planning Portal’s summary limits a single-storey rear extension to 3 metres beyond the rear wall of the original house, or 4 metres if detached, rising to 6 and 8 metres under prior approval, with a 4 metre height limit and no more than half the land around the original house covered. Flats and maisonettes have no such rights, designated land such as conservation areas is more restricted, and an Article 4 direction can remove the rights. A lawful development certificate confirms the position and is not compulsory.

Building Regulations approval

Most extensions need Building Regulations approval, from the council’s building control or a registered building control approver. A full plans application gets a decision within 5 weeks, or 2 months with consent. A building notice is for smaller projects and work can start 2 days after it is submitted, but with no approved plans the inspector’s requirements arrive on site rather than in your price. Each council sets its charges in its own charging scheme under the Building (Local Authority Charges) Regulations 2010, so get the figure for that council.

Party wall notices

The Party Wall etc. Act 1996 applies in England and Wales. Notice of work to a party wall or party structure goes at least two months before the work begins, under section 3. A new wall on the boundary line, or an excavation within 3 metres of a neighbour’s building that goes below their foundations (or within 6 metres where it cuts a 45 degree line down from them), needs at least one month’s notice under sections 1 and 6. The neighbour has 14 days to respond in writing. For party structure and excavation notices, silence counts as a dispute, which means surveyors and a party wall award, and the award sets who pays the surveyors. For a new wall at the line of junction, silence means the wall goes wholly on the client’s own land. See party wall notices.

CDM 2015 and the Building Regulations dutyholders

A homeowner is a domestic client. Under regulation 7 of the Construction (Design and Management) Regulations 2015 the client duties pass to the contractor, or to the principal contractor where there is more than one contractor, and a construction phase plan is needed however small the job. Regulation 11C of the Building Regulations 2010 does the same for the Part 2A dutyholder duties. Either way it is your time, and it belongs in the price.

NoteWales has its own planning rules and its own amendments to the Building Regulations, and Scotland and Northern Ireland run separate building control systems. The Party Wall Act covers England and Wales only.

How do you break an extension into stages?

Price it in the order it is built. Each stage is a heading you can measure, cost and later check against, and the client can see what each part of the money buys.

01Set-up and preliminaries. Welfare, protection, skips, scaffold and supervision. Mostly time-related, so they follow the programme.
02Demolition and opening up. The old conservatory or patio, breaking out, and finding out what is there.
03Substructure and drainage. Excavation, spoil, foundations, walls to DPC, the ground floor and the drains.
04Superstructure and roof. Walls, lintels, roof structure and coverings, rainwater goods, windows and doors.
05Openings and steels into the existing house. Temporary support, the opening, padstones and steels.
06First fix. Carpentry, electrics, plumbing and heating before the walls are closed.
07Plastering. Boarding, plaster and screed.
08Second fix. Doors, skirtings, sockets, radiators and sanitary ware.
09Finishes. Decoration, tiling and floor finishes.
10Making good. Where the old meets the new, inside and out.
11External works. Paths, the patio, soakaways and reinstating the garden.

Measure each stage before you price it: see how to do a quantity take-off from drawings. A figure for blockwork with no area behind it cannot be checked.

How do you price the labour, materials, plant and subcontractors?

Labour

At its true cost per productive hour, not the wage: holiday, employer’s National Insurance, pension, non-productive time, training and insurance all sit on top. Estimate the hours for each stage from your own records of similar work. The method is in how to price a job properly.

Materials

From the take-off, at current merchant prices, with the waste allowance in the rate rather than in the measured quantity. Add delivery, ask the merchant to hold prices for as long as your quote is valid, and check lead times on steels and windows.

Plant and waste

Digger, dumper, breaker, props, scaffold and welfare, priced for the days the programme needs, idle days included. Spoil takes up more room out of the ground than in it, so allow for that when pricing skips or grab lorries.

Subcontract trades

Electrician, plumber, plasterer, roofer and the window installer. Get written quotes against the same drawings and specification, read what each excludes, and add your attendance and margin. A subcontract quote that excludes making good leaves it with you.

Which costs do builders forget?

The ones outside the build. Agree who pays each, and if it is the client, say so in the exclusions.

Building control charges, from the council’s charging scheme or the approver’s quote.
The structural engineer, for the calculations and any return visit.
Party wall surveyors, where a notice is served and the neighbour does not consent.
Planning or lawful development certificate fees, where nobody has applied yet.
Surveys. An asbestos survey before opening up an older house, and a drainage survey where the runs are unknown.
Skip licences where the skip stands on the road. In most areas the skip company gets the licence, so check it is in their price.
Your own time on the paperwork. The construction phase plan, the notices, the building control inspections and the handover documents.

Provisional sums and risk

Some of the job cannot be seen until you dig. Price what you can, and for the rest use a provisional sum: a stated allowance for work that cannot yet be fully described, adjusted to the real cost once it is known. On an extension the usual candidates are foundations deeper than designed, a drain diversion and an unknown condition in the existing wall. State the basis, for example foundations priced to the depth on the drawing and anything deeper at a stated rate per cubic metre. See provisional sums and PC sums.

Risk you choose to carry, such as a tight programme around a family living in the house, belongs in your own build-up as a contingency.

Can you show me a worked example?

A single-storey rear extension of about 4 by 6 metres, with a steel into the existing rear wall and a set of bifold doors. Every figure here is made up to show the method.

Stage	Labour	Materials	Plant and hire	Subcontract	Stage cost
Set-up and preliminaries	£1,400	£300	£2,100	£0	£3,800
Demolition and opening up	£900	£0	£400	£0	£1,300
Substructure and drainage	£2,600	£2,900	£1,100	£0	£6,600
Superstructure and roof	£4,800	£6,200	£300	£2,400	£13,700
Openings and steels	£1,400	£1,600	£200	£0	£3,200
First fix	£900	£600	£0	£2,200	£3,700
Plastering	£0	£0	£0	£2,100	£2,100
Second fix	£800	£700	£0	£1,300	£2,800
Finishes	£1,100	£900	£0	£0	£2,000
Making good	£600	£200	£0	£0	£800
External works	£900	£700	£200	£0	£1,800
Direct cost	£15,400	£14,100	£4,300	£8,000	£41,800

Illustrative figures for a worked example, not market rates. Price from your own rates, take-off and subcontract quotes.

Step	Calculation	Amount
Direct cost	From the stage table	£41,800
Overhead recovery	440 productive hours × £20	£8,800
Cost plus overhead	£41,800 + £8,800	£50,600
Margin at 12% of the price	£50,600 ÷ 0.88, less £50,600	£6,900
Price before VAT		£57,500
VAT at 20%	Where you are VAT-registered	£11,500
Price including VAT		£69,000

Illustrative figures. The overhead rate comes from the firm’s own accounts; margin is taken as a share of the price.

The 440 hours are the firm’s own labour: £15,400 at a true cost of £35 an hour. Overhead is recovered before margin is added, and 12% of the price needs a markup of about 13.6% on cost plus overhead. Put 12% on the cost instead and the price comes out about £800 short. See markup and margin.

Free toolMarkup vs margin calculatorConvert between markup and margin, and see the price a target margin needs.

VAT goes on the whole price. HMRC’s VAT Notice 708 says work to an existing building is normally standard-rated, and an extension is zero-rated only to the extent that it creates an additional dwelling. If you are VAT-registered, show the client the VAT-inclusive total as plainly as the net one.

How should you set the payment stages and exclusions?

A contract with a homeowner for work on their own home is outside the payment rules of the Housing Grants, Construction and Regeneration Act 1996: section 106 excludes contracts with a residential occupier. There are no statutory payment notices to fall back on, so the payment terms in your quote are the whole arrangement. Tie each stage payment to an event both sides can see, not to a date.

Stage payment	Due when
Deposit	The quote is accepted, covering materials and long-lead items ordered for the job
Substructure	Foundations poured and inspected, drains tested
Shell	Roof on and the extension weathertight
Opening	Steel in and the opening formed
Plastering	First fix and plastering complete
Completion	Practical completion, with the snagging list agreed

For deposits, invoicing and chasing, see getting paid faster. If the contract is agreed in the client’s home, the Consumer Contracts Regulations 2013 may give them a right to cancel: see can a customer cancel building work?

The written exclusions

Planning, building control and lawful development certificate fees, unless included.
Structural engineer’s and party wall surveyors’ fees.
Foundations deeper than the drawing, beyond the provisional sum.
Removal of asbestos or other hazardous material found when opening up.
Diverting drains or gas, electricity or water supplies beyond what is drawn.
Kitchen units, appliances, floor coverings, light fittings or sanitary ware supplied by the client.
Any work on the neighbour’s side of the boundary.

Most arguments about the price of an extension are arguments about scope. An exclusion written into the quote settles them before they start.

How does Estimark help you price an extension?

Estimark builds the estimate from your own price book: your own items priced the way you buy them, favourites for the lines on every job, and assemblies for the groups you price as one thing. Labour rates per trade, overhead recovery and margin are set once for the company, so two people pricing the same extension land in the same place, and an old estimate keeps the rates it was built on. Materials, labour, plant and subcontract stay in separate categories, and a take-off holds each measurement as typed dimensions and puts the lines on a quote. See tenders and estimating.

The estimate becomes the quote: a branded PDF with a valid-until date that the client opens from a link, then accepts and signs online without creating an account. The accepted quote converts into a job carrying the accepted total as its budget, so the price you agreed is the figure the job is measured against. See quotes and online acceptance.

Sources

Questions

Frequently asked

How much does a house extension cost per square metre?

There is no reliable single rate. Ground, the steel into the house, drainage, access and the specification move the cost far more than the floor area does. Build the price stage by stage, and use a rate per square metre from your own past jobs only as a check.

Is VAT charged on a house extension?

Yes, at the standard rate of 20% where the builder is VAT-registered. HMRC’s VAT Notice 708 treats work to an existing building as standard-rated, and an extension is zero-rated only to the extent that it creates an additional dwelling.

Does a single-storey extension need planning permission?

Not always. In England many single-storey rear extensions are permitted development within limits on depth, height and the share of the plot covered, and a lawful development certificate confirms it. Flats, designated land and Article 4 areas are different. Building Regulations approval is a separate question, and most extensions need it.

Who pays the party wall surveyor?

The building owner, the homeowner, pays for the building work. Where surveyors are appointed, the party wall award sets who pays their fees. Put party wall costs in your exclusions or as a stated allowance, never as an assumption.

Do I need a construction phase plan for a domestic extension?

Yes. CDM 2015 requires one on every project, and on a domestic job the client’s duties pass to you as the contractor under regulation 7, or to the principal contractor if there is more than one contractor.

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