House extension stages, costs and timings: what to expect
A house extension typically runs through seven stages, from groundworks to snagging, over 12 to 16 weeks on site. UK build costs range from £38,000 to £95,000, or £52,000 to £135,000 for a full project including fit-out and VAT. Payment tied to each stage, rather than paid upfront, protects both sides.
7
Stages
£52,000–£135,000
Typical UK cost
12 to 16 weeks on site
On-site duration
What are the stages of house extensions?
House extensions typically break into 7 stages. Each stage has a share of the total budget and its own risks, and paying against each one as it completes, rather than upfront, protects both sides.
Stage 1: Mobilisation, site set-up and enabling works
5% of budget · £2,600-£6,800 · 1 week
Skips, fencing and scaffold arrive, services are located, and any existing structure is cleared before setting out begins.
What goes wrong
What it costs
How you check
- What goes wrong
- Work starts with no signed contract, drawing register or itemised specification.
- What it costs
- Every later change becomes a negotiation with no baseline. A typical unresolved scope dispute costs £2,000 to £8,000 and stalls the project 2 to 4 weeks.
- How you check
- Have a signed contract, dated drawing register and itemised specification in hand before any money moves.
- What goes wrong
- A large deposit is requested up front, framed as materials procurement.
- What it costs
- 20 to 30% of the contract value is exposed if the builder fails or walks away, with little chance of recovering it from a dissolved company.
- How you check
- Cap this stage at around 5% of the contract. Materials are ordered against later stages when they are actually needed.
- What goes wrong
- Public liability and contractors all risks insurance are not in place.
- What it costs
- Damage to the neighbouring property or the existing house is uninsured, typically £5,000 or more, and the cost falls on the homeowner.
- How you check
- Ask for a certificate of public liability insurance at £2 million minimum, in the trading name on the contract, and check it is in date.
- What goes wrong
- Party wall notices are not served where the law requires them.
- What it costs
- A neighbour can seek an injunction stopping work, losing 4 to 8 weeks, and the surveyor costs of £1,000 to £2,500 per neighbour are incurred anyway.
- How you check
- Get a signed party wall award, or written neighbour consent on the statutory form, before work starts.
- What goes wrong
- The planning route is not confirmed, and work proceeds on an assumption of permitted development.
- What it costs
- An enforcement notice can follow, and in the worst case completed work has to be demolished.
- How you check
- Get a Lawful Development Certificate or a planning decision notice. A builder’s opinion is not proof.
Stage 2: Groundworks, foundations and drainage
18% of budget · £9,500-£24,500 · 2 to 3 weeks
Trenches are dug to firm ground, inspected by building control, and filled with concrete, and new drainage is laid before the floor slab goes in.
What goes wrong
What it costs
How you check
- What goes wrong
- Foundations have to go deeper than quoted, because of clay, nearby trees, made ground or an old cellar.
- What it costs
- This is the single most common extension overrun: typically £2,000 to £8,000 of extra concrete and labour, and 1 to 2 weeks added.
- How you check
- Agree a provisional sum in writing before work starts, stating the assumed depth and the rate per additional 300mm.
- What goes wrong
- A live drain or shared sewer is found running under the footprint.
- What it costs
- £1,500 to £5,000 to divert it, or a build-over agreement with the water authority costing £300 to £600 and taking 3 to 6 weeks.
- How you check
- Get a drainage survey or water authority asset plan before excavation, and the build-over agreement issued before concrete is poured.
- What goes wrong
- Concrete is poured before building control has inspected the open trench.
- What it costs
- The inspector can require it to be dug up again, costing £3,000 to £10,000 and 2 weeks, with no guarantee it will pass.
- How you check
- Ask for the building control inspection record for the trench, dated, before the concrete is poured.
- What goes wrong
- The damp proof membrane is omitted, punctured, or not properly lapped into the damp proof course.
- What it costs
- Rising damp through the new slab, which usually means lifting the finished floor to fix. Typically £2,000 to £6,000.
- How you check
- Ask for dated photographs of the membrane laid, lapped and turned up at the edges, before the slab is poured.
Stage 3: Superstructure, walls and structural opening
20% of budget · £10,500-£27,000 · 2 to 3 weeks
Cavity walls are built up, the floor slab is poured, and the structural opening into the existing house is formed with a steel beam.
What goes wrong
What it costs
How you check
- What goes wrong
- The steel beam is installed without a structural engineer’s calculation pack, or a smaller section is substituted.
- What it costs
- Deflection or cracking above the opening, and in serious cases structural failure. Remedial work costs £5,000 to £15,000 and often means removing the beam through a finished room.
- How you check
- Ask for the engineer’s calculation pack matching the beam actually installed, plus the building control inspection record for the steel.
- What goes wrong
- Padstones are omitted or undersized, so the beam bears directly onto blockwork.
- What it costs
- Point loading crushes the blockwork over months, and cracking appears after the builder has left.
- How you check
- Ask for photographs of the bearings before the opening is closed up and plastered.
- What goes wrong
- Cavity insulation is gapped, wall ties are spaced incorrectly, or cavity trays are missed where a new roof meets the existing wall.
- What it costs
- Damp bridging into the new room and a failed insulation check at completion, typically £2,000 or more to open up and correct.
- How you check
- Ask for photographs of the cavity before it closes, plus the building control inspection at damp proof course and pre-plaster stage.
- What goes wrong
- Built dimensions drift from the drawings, most often the opening width or the internal clear span.
- What it costs
- Pre-ordered glazing or kitchen units no longer fit, costing £3,000 to £10,000 to reorder, with glazing lead times of 6 to 10 weeks.
- How you check
- Get a measured check against the drawings before the roof goes on, and do not order glazing until that check is done.
Stage 4: Roof, glazing and weathertight envelope
17% of budget · £9,000-£23,000 · 2 to 3 weeks
The roof structure, covering, windows and external doors go on, and the extension becomes a watertight shell.
What goes wrong
What it costs
How you check
- What goes wrong
- Glazing and rooflights are not ordered early enough.
- What it costs
- The shell sits open and idle for 6 to 10 weeks while scaffold hire continues at £150 to £400 a week and the whole programme slips.
- How you check
- Get an order confirmation with a delivery date sighted at the end of the previous stage, not this one.
- What goes wrong
- The flat roof is laid to a shallow fall, or no fall at all.
- What it costs
- Ponding, then membrane failure. Typically £3,000 to £6,000 to strip and relay, often 18 months later when the guarantee argument starts.
- How you check
- Ask for a hose test, or watch after a full day of rain, before releasing payment, and register the roof covering guarantee in your own name.
- What goes wrong
- The builder describes the shell as watertight while flashings, upstands or rainwater goods are still incomplete.
- What it costs
- Water reaches new plaster and first fix electrics, meaning £2,000 to £8,000 of rework and 3 to 4 weeks of drying out.
- How you check
- Physically inspect after rain, and ask for photographs of every abutment, upstand and flashing detail.
- What goes wrong
- Windows are fitted without trickle vents, or the wrong glazing specification is supplied against the approved drawings.
- What it costs
- A ventilation failure at completion, with sashes replaced at £400 to £900 each.
- How you check
- Check the FENSA or CERTASS certificate and manufacturer spec sheet against the approved drawings.
Stage 5: First fix, insulation, screed and plastering
15% of budget · £8,000-£20,500 · 2 to 3 weeks
Cables, pipework and studwork go in, insulation and screed are laid, and everything is covered permanently by plaster.
What goes wrong
What it costs
How you check
- What goes wrong
- Socket, switch and light positions are never confirmed with the homeowner before boarding.
- What it costs
- Chasing out finished walls afterwards costs £800 to £2,500 and leaves patched decoration.
- How you check
- Walk round with the electrician while the first fix is exposed, and mark and photograph the agreed positions.
- What goes wrong
- Underfloor heating is screeded over without a pressure test.
- What it costs
- A single pinhole leak means lifting the screed and finished floor, typically £2,000 to £5,000.
- How you check
- Ask for a dated pressure test certificate, with the gauge photographed holding pressure.
- What goes wrong
- Insulation is short of the specified thickness, gapped, or compressed around services.
- What it costs
- A failed insulation check at completion and a cold room for the life of the building. Opening up to fix typically costs £2,000 or more.
- How you check
- Ask for photographs of every wall, ceiling and floor zone before boarding, with the product label visible.
- What goes wrong
- Electrics are plastered over by someone who cannot self-certify the work.
- What it costs
- No electrical installation certificate at completion, meaning retrospective inspection and testing at £400 to £900, and possibly rewiring.
- How you check
- Check the electrician’s scheme registration number on the scheme’s own register before boarding starts.
Stage 6: Second fix, fit-out and decoration
20% of budget · £10,500-£27,000 · 3 to 4 weeks
Sockets, radiators and doors are fitted, the kitchen or bathroom goes in, and floors are laid and decorated.
What goes wrong
What it costs
How you check
- What goes wrong
- The builder starts the next job and thins the crew to one or two people.
- What it costs
- The last 10% of the work drags on 4 to 8 weeks past the agreed date. This is the most common single complaint in UK domestic building.
- How you check
- Make sure enough contract value sits in the final two stages that finishing this job is worth more to the builder than leaving it.
- What goes wrong
- Homeowner-supplied kitchen, tiles or sanitaryware arrive late or damaged.
- What it costs
- 2 to 4 weeks of standing time, and the builder can reasonably claim a delay cost.
- How you check
- Get all homeowner-supplied items delivered and checked on site before the stage starts.
- What goes wrong
- There is no agreed standard for what "finished" decoration means.
- What it costs
- An unresolvable argument over paint coverage, caulking, tiling tolerance and joinery gaps, which is where most snagging disputes start.
- How you check
- Agree a written finish standard, or photograph a benchmark area early and reference it in the contract.
Stage 7: External works, snagging and handover
5% of budget · £2,600-£6,800 · 1 to 2 weeks
The patio and garden are made good, scaffold comes down, and the snagging list is worked through and signed off before certificates are handed over.
What goes wrong
What it costs
How you check
- What goes wrong
- The Building Regulations completion certificate is never obtained.
- What it costs
- The extension becomes unmortgageable and blocks a future sale. Retrospective regularisation costs £500 to £2,000 plus the cost of opening up finished work for inspection. This is the single most damaging omission on the whole project.
- How you check
- Get the certificate itself in your hands, not just the builder’s assurance that it is coming.
- What goes wrong
- Certificates for electrics, gas, glazing and structure are never collected.
- What it costs
- Each one costs £300 to £900 to recreate retrospectively, if it can be recreated at all.
- How you check
- Ask for a handover pack containing the electrical installation certificate, Gas Safe certificate if applicable, FENSA or CERTASS for the glazing, the structural engineer’s sign-off, and roof and glazing guarantees registered in your name.
- What goes wrong
- The snagging list is closed under pressure with items still open.
- What it costs
- Once the final payment is released the leverage is gone. Typical residual snagging cost picked up by the homeowner is £500 to £2,500.
- How you check
- Keep a dated, photographed snagging list, item by item, each marked closed with a photograph, and only sign off when every line is closed.
- What goes wrong
- Scaffold is left standing on a homeowner-held hire account.
- What it costs
- £150 to £400 a week of avoidable cost.
- How you check
- Name scaffold hire as the builder’s cost in the contract, with a strike date recorded.
How much does a house extension cost in the UK?
Typical UK costs range from £52,000 to £135,000. Full project budget for a single storey rear extension of 15 to 30 square metres, mid spec, including fees, fit-out and VAT, UK national average.
The two largest stages by budget, superstructure, walls and structural opening and second fix, fit-out and decoration, account for around 40% of the total between them.
| Typical UK range | £52,000-£135,000 |
|---|---|
| Per square metreSingle storey rear, mid spec, ex VAT, UK national average. Rates flatten above roughly 30 square metres. Below 15 square metres they rise sharply, because scaffold, welfare, skips and setting up are close to fixed regardless of size. | £1,900-£3,000/m² |
| Single storey rear | £38,000-£95,000 |
| Double storeyRoughly 40 square metres. Cheaper per square metre than single storey, higher total because you are buying twice the floor area. | £85,000-£160,000 |
| Side return | £2,200-£3,300/m² |
| Wraparound | £2,100-£3,100/m² |
| Shell only, no internal finishes | £1,300-£1,900/m² |
- London and South East:
- +20-40%
- Midlands:
- 0%
How long do house extensions take?
On-site work typically takes 12 to 16 weeks on site. Allowing for the delays most projects see, budget on 14 to 20 weeks allowing for typical delays.
How should you pay a builder for a house extension?
Pay against each stage as it completes, not upfront. 5% of the total contract value is held from the final stage and released once the snagging list is signed off and the Building Regulations completion certificate is in the homeowner’s hands.
With Renopay, the money for each stage is held securely and released once that stage is signed off, so the builder has proof of funds before starting and you only pay for work that is actually done.
- Watertight shell
- The point at which walls, roof covering, windows and external doors are all in place so rain can no longer get into the structure.
- Party wall award
- A legal document prepared by surveyors setting out how work affecting a shared wall or boundary may proceed, and recording the neighbouring property’s condition before work starts.
- Permitted development
- The right to build certain extensions without a full planning application, subject to size, height and siting limits.
- Lawful Development Certificate
- A council-issued document confirming work falls within permitted development. This is the proof a buyer’s solicitor will ask for at sale.
- Trench fill foundation
- The standard extension foundation: a trench dug to firm ground and filled almost to the top with concrete.
- Damp proof course and damp proof membrane
- The barriers that stop ground moisture rising into the building: a damp proof course in the walls, and a damp proof membrane under the floor slab.
- Oversite
- The prepared ground beneath a new floor slab, including compacted hardcore, sand blinding, membrane and insulation.
- Structural opening
- The new hole formed in the existing external wall to connect old and new, usually supported by a steel beam.
- Steel beam (RSJ)
- The rolled steel section carrying the load over a structural opening. Its size must come from a structural engineer’s calculations.
- Padstone
- The concrete or engineering brick block that spreads a steel beam’s load into the wall beneath it.
- Cavity tray
- A formed barrier inside a cavity wall that catches water and directs it back out, typically where a new roof meets an existing wall.
- First fix and second fix
- First fix is everything installed before plastering, such as cables, pipes and studwork. Second fix is everything fitted after, such as sockets, radiators, doors and sanitaryware.
- Screed
- The levelling layer poured over insulation or underfloor heating before the final floor finish goes down.
- Provisional sum
- A placeholder amount in a quote for work that cannot be priced accurately until it is opened up, most often foundation depth.
- Snagging
- The process of listing and fixing minor defects once the work is functionally complete.
- Building Regulations completion certificate
- The council or approved inspector’s final sign-off confirming the work meets the regulations. A buyer’s solicitor will require it.
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