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–£135,000·12 to 16 weeks on site typical
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 · 1 week
Skips, fencing and scaffold arrive, services are located, and any existing structure is cleared before setting out begins.
Scaffold goes up where needed and welfare facilities arrive on site. Services are located and isolated, and any existing structure such as a conservatory or outbuilding is demolished and cleared.
Setting out is marked on the ground against the approved drawings before any groundwork starts.
What goes wrong at this stage
- Work starts with no signed contract, drawing register or itemised specification.
- 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.
- Protects against it: Have a signed contract, dated drawing register and itemised specification in hand before any money moves.
- A large deposit is requested up front, framed as materials procurement.
- 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.
- Protects against it: Cap this stage at around 5% of the contract. Materials are ordered against later stages when they are actually needed.
- Public liability and contractors all risks insurance are not in place.
- Damage to the neighbouring property or the existing house is uninsured, typically £5,000 or more, and the cost falls on the homeowner.
- Protects against it: 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.
- Party wall notices are not served where the law requires them.
- 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.
- Protects against it: Get a signed party wall award, or written neighbour consent on the statutory form, before work starts.
- The planning route is not confirmed, and work proceeds on an assumption of permitted development.
- An enforcement notice can follow, and in the worst case completed work has to be demolished.
- Protects against it: Get a Lawful Development Certificate or a planning decision notice. A builder’s opinion is not proof.
Keep this stage small, around 5% of the contract. The money is held securely and released once the signed contract, insurance certificate and any required party wall award are all confirmed.
Stage 2: Groundworks, foundations and drainage
18% of budget · 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.
Trenches are excavated to firm bearing ground, inspected by building control, and filled with concrete. Existing drainage is diverted or built over, and new drainage runs and inspection chambers are installed.
The oversite is prepared with hardcore, membrane and insulation, ready for the floor slab.
What goes wrong at this stage
- Foundations have to go deeper than quoted, because of clay, nearby trees, made ground or an old cellar.
- 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.
- Protects against it: Agree a provisional sum in writing before work starts, stating the assumed depth and the rate per additional 300mm.
- A live drain or shared sewer is found running under the footprint.
- £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.
- Protects against it: Get a drainage survey or water authority asset plan before excavation, and the build-over agreement issued before concrete is poured.
- Concrete is poured before building control has inspected the open trench.
- 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.
- Protects against it: Ask for the building control inspection record for the trench, dated, before the concrete is poured.
- The damp proof membrane is omitted, punctured, or not properly lapped into the damp proof course.
- Rising damp through the new slab, which usually means lifting the finished floor to fix. Typically £2,000 to £6,000.
- Protects against it: Ask for dated photographs of the membrane laid, lapped and turned up at the edges, before the slab is poured.
Payment for this stage is held securely and released once building control has inspected and signed off the foundations and drainage, not simply once concrete has been poured.
Stage 3: Superstructure, walls and structural opening
20% of budget · 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.
Cavity walls are built from the damp proof course to wall plate height, with insulation, ties and cavity trays. The floor slab is poured.
The structural opening into the existing house is formed and the steel beam is installed onto padstones and propped until it takes load.
What goes wrong at this stage
- The steel beam is installed without a structural engineer’s calculation pack, or a smaller section is substituted.
- 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.
- Protects against it: Ask for the engineer’s calculation pack matching the beam actually installed, plus the building control inspection record for the steel.
- Padstones are omitted or undersized, so the beam bears directly onto blockwork.
- Point loading crushes the blockwork over months, and cracking appears after the builder has left.
- Protects against it: Ask for photographs of the bearings before the opening is closed up and plastered.
- Cavity insulation is gapped, wall ties are spaced incorrectly, or cavity trays are missed where a new roof meets the existing wall.
- Damp bridging into the new room and a failed insulation check at completion, typically £2,000 or more to open up and correct.
- Protects against it: Ask for photographs of the cavity before it closes, plus the building control inspection at damp proof course and pre-plaster stage.
- Built dimensions drift from the drawings, most often the opening width or the internal clear span.
- 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.
- Protects against it: Get a measured check against the drawings before the roof goes on, and do not order glazing until that check is done.
This is one of the largest stages by value, reflecting the structural work involved. Payment is held securely and released once the engineer’s sign-off and building control inspection for the steel are confirmed.
Stage 4: Roof, glazing and weathertight envelope
17% of budget · 2 to 3 weeks
The roof structure, covering, windows and external doors go on, and the extension becomes a watertight shell.
The roof structure goes on, followed by the covering, flashings, upstands and rainwater goods. Windows, rooflights and external doors are installed.
At the end of this stage the extension should be fully watertight, with no route for rain to get in.
What goes wrong at this stage
- Glazing and rooflights are not ordered early enough.
- 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.
- Protects against it: Get an order confirmation with a delivery date sighted at the end of the previous stage, not this one.
- The flat roof is laid to a shallow fall, or no fall at all.
- Ponding, then membrane failure. Typically £3,000 to £6,000 to strip and relay, often 18 months later when the guarantee argument starts.
- Protects against it: 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.
- The builder describes the shell as watertight while flashings, upstands or rainwater goods are still incomplete.
- Water reaches new plaster and first fix electrics, meaning £2,000 to £8,000 of rework and 3 to 4 weeks of drying out.
- Protects against it: Physically inspect after rain, and ask for photographs of every abutment, upstand and flashing detail.
- Windows are fitted without trickle vents, or the wrong glazing specification is supplied against the approved drawings.
- A ventilation failure at completion, with sashes replaced at £400 to £900 each.
- Protects against it: Check the FENSA or CERTASS certificate and manufacturer spec sheet against the approved drawings.
This is what "watertight shell" refers to on the payment schedule. Money for this stage is held securely and released once the extension can genuinely keep out rain, confirmed by inspection after wet weather, not just by the builder’s word.
Stage 5: First fix, insulation, screed and plastering
15% of budget · 2 to 3 weeks
Cables, pipework and studwork go in, insulation and screed are laid, and everything is covered permanently by plaster.
Cables, pipework, underfloor heating and studwork are installed. Insulation is fitted to the roof and floor, and screed is laid.
Everything is then covered permanently by plasterboard and plaster, so this is the last chance to check work that will otherwise be hidden.
What goes wrong at this stage
- Socket, switch and light positions are never confirmed with the homeowner before boarding.
- Chasing out finished walls afterwards costs £800 to £2,500 and leaves patched decoration.
- Protects against it: Walk round with the electrician while the first fix is exposed, and mark and photograph the agreed positions.
- Underfloor heating is screeded over without a pressure test.
- A single pinhole leak means lifting the screed and finished floor, typically £2,000 to £5,000.
- Protects against it: Ask for a dated pressure test certificate, with the gauge photographed holding pressure.
- Insulation is short of the specified thickness, gapped, or compressed around services.
- 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.
- Protects against it: Ask for photographs of every wall, ceiling and floor zone before boarding, with the product label visible.
- Electrics are plastered over by someone who cannot self-certify the work.
- No electrical installation certificate at completion, meaning retrospective inspection and testing at £400 to £900, and possibly rewiring.
- Protects against it: Check the electrician’s scheme registration number on the scheme’s own register before boarding starts.
Everything in this stage gets covered up permanently, so this payment is held securely and released only once you have seen and photographed the work while it is still exposed, not after the walls close.
Stage 6: Second fix, fit-out and decoration
20% of budget · 3 to 4 weeks
Sockets, radiators and doors are fitted, the kitchen or bathroom goes in, and floors are laid and decorated.
Sockets, switches, radiators, doors, skirting and architrave go in. Kitchen or bathroom is fitted.
Floor finishes are laid and the space is decorated to completion.
What goes wrong at this stage
- The builder starts the next job and thins the crew to one or two people.
- 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.
- Protects against it: Make sure enough contract value sits in the final two stages that finishing this job is worth more to the builder than leaving it.
- Homeowner-supplied kitchen, tiles or sanitaryware arrive late or damaged.
- 2 to 4 weeks of standing time, and the builder can reasonably claim a delay cost.
- Protects against it: Get all homeowner-supplied items delivered and checked on site before the stage starts.
- There is no agreed standard for what "finished" decoration means.
- An unresolvable argument over paint coverage, caulking, tiling tolerance and joinery gaps, which is where most snagging disputes start.
- Protects against it: Agree a written finish standard, or photograph a benchmark area early and reference it in the contract.
This stage carries real weight deliberately, so finishing the job is worth more to the builder than moving on. Money is held securely and released once each element is signed off against the agreed finish standard.
Stage 7: External works, snagging and handover
5% of budget · 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.
Patio, paths, drainage gullies and making good of the garden. Scaffold comes down and the site is given a final clean.
The snagging list is walked, worked through and signed off, and certificates are issued and handed over.
What goes wrong at this stage
- The Building Regulations completion certificate is never obtained.
- 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.
- Protects against it: Get the certificate itself in your hands, not just the builder’s assurance that it is coming.
- Certificates for electrics, gas, glazing and structure are never collected.
- Each one costs £300 to £900 to recreate retrospectively, if it can be recreated at all.
- Protects against it: 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.
- The snagging list is closed under pressure with items still open.
- Once the final payment is released the leverage is gone. Typical residual snagging cost picked up by the homeowner is £500 to £2,500.
- Protects against it: Keep a dated, photographed snagging list, item by item, each marked closed with a photograph, and only sign off when every line is closed.
- Scaffold is left standing on a homeowner-held hire account.
- £150 to £400 a week of avoidable cost.
- Protects against it: Name scaffold hire as the builder’s cost in the contract, with a strike date recorded.
This final 5% is the holdback. It is released only once the snagging list is fully signed off and the Building Regulations completion certificate is in your hands, not on the builder’s assurance that it is coming.
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. Costs typically run 20 to 40% higher in London and the South East than the UK average.
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.
Terms you’ll hear on site
- 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.
Questions
Book a 30-minute call
Book a 30-minute call
For builders and architects exploring Renopay. Pick a time that works.
Prefer email?
Drop us a line at hello@renopay.co.uk and we will set up a call.