Cost My Extension

Single-storey rear extension

Address: Sample house, Kensal Rise, NW10 3XX
Borough: Brent
Written: 20 October 2026
Pricing basis: 2026-10-20.1
Prepared for: Sample Architects LLP

Expected cost: building work, fit-out and fees, VAT included

£120,500

Likely between £95,500 to £146,000. Building work alone £52,000 to £85,500 for about 20 m².

Expected £120,500

£95,500£146,000

Budget to have available: £154,500; The top of the likely range plus £8,000 contingency to hold back.
Planning: Larger home extension: prior approval needed
Time: 23 to 43 weeks; From today to moving back in, 8 to 16 of them on site.
Worth knowing about: Party wall noticesPublic sewer nearby (to check); Each is explained under planning and permissions.

For your client's 20 m² single-storey rear extension with kitchen and bifolds, the all-in figure is £92,500 to £165,500 inc VAT (ex-VAT figures sit beneath each line in the tables), with £129,000 the likely outturn. Build is £52,000 to £85,500; the balance is fees, the council's prior approval fee and a contingency. At 4 m the scheme sits in the larger home extension route rather than standard permitted development, and both party walls are engaged. Programme 23 to 43 weeks from survey to completion certificate.

The house

EPC: 1900 to 1929 mid-terrace, about 94 m², no extension recorded, rating D. Planning register: no conservation area at the postcode; the only Article 4 direction in Brent is the HMO one (Part 3 Class L), so Part 1 rights are intact. Flood zone 1. The constraint checks run on the postcode centroid, so confirm against the council's map at the plot before relying on them.

House: Mid-terrace house
On the EPC register: Mid-Terrace, House
Floor area on record: 94 m²
Age band: 1900-1929
Conservation area: No
Article 4 direction: None affecting extensions
Flood zone: Zone 1

Plan, to scale. Your extension in red, 4 m deep by 5 m wide.

Checked: postcodes.io (ok); EPC register (ok); planning.data.gov.uk (ok). Constraint checks use the postcode's centre point, so they describe the area, not the exact plot.

The cost

inc VAT

Where the expected £120,500 goes, with the £8,000 contingency beside it: money to have available, not money you will certainly spend.

Build £52,000 to £85,500 for 20 m², about £3,450/m² at Brent's factor, from the itemised schedule: the governing items are the steel across the rear elevation, strip foundations at London clay depth, the cavity walls and the warm-deck roof. Extras priced outside the schedule: kitchen £16,000 to £28,000, bifolds £7,000 to £11,000, the opening into the existing house £4,500 to £8,000. Fees: design £4,200 to £11,100, engineer £1,000 to £2,500, building control £1,000 to £1,800, party wall up to £0 to £6,000 across two neighbours if both dissent. Contingency £6,200 to £10,300 at 12% of build. Sensitivity: high spec £45,000; basic £19,500; 3 m depth £10,500, which also returns the scheme to Class A without prior approval.

Building work

Extension: groundworks, structure, roof, glazing, first and second fixAbout 20 m². Broken down by trade under where the money goes.

£52,000 to £85,400£43,400 to £71,200 ex VAT

Fit-out you chose

New kitchen: units, worktops, appliances, fitting	£16,000 to £28,000£13,300 to £23,300 ex VAT
Bifold or sliding doors	£7,000 to £11,000£5,800 to £9,200 ex VAT
Knocking through: steel beam, making good	£4,500 to £8,000£3,800 to £6,700 ex VAT

Fees and approvals

Architect or designer: drawings, planning and building regulations package	£4,200 to £11,100£3,500 to £9,300 ex VAT
Structural engineer: calculations and beam design	£1,000 to £2,500£800 to £2,100 ex VAT
Council prior approval feeStatutory fee plus the Planning Portal's service charge.	£334£278 ex VAT
Building control: plan check and inspections	£1,000 to £1,800£800 to £1,500 ex VAT
Party wall surveyors (2 neighbours, if they dissent)Zero if your neighbours consent in writing. Budget the top figure if relations are cool.	£0 to £6,000£0 to £5,000 ex VAT
Thames Water build-over agreement (if a public sewer is within 3 m)	£0 to £1,200£0 to £1,000 ex VAT

Expected costLikely £95,500 to £146,000	£120,500£100,500 ex VAT
Contingency to hold back (12% of the building work)Not a cost you will certainly pay: money to have available if something unforeseen turns up.	£8,000£7,000 ex VAT
Budget to have availableThe top of the likely range plus the contingency.	£154,500£128,500 ex VAT

The likely range allows for the lines moving partly together, as they do on real jobs. If every line came in at its cheapest at once the total would be £86,000; at its dearest, £155,500. All figures are shown inc VAT, with the ex-VAT figure beneath (inc VAT divided by 1.2).

Where the money goes

inc VAT

The building work, £52,000 to £85,500, split by trade from your dimensions and finish, priced for Brent. The percentage is each trade's share of the building work, taken at the middle of its range. Open any line for the quantities and rates behind it.

Preliminaries7%£3,600 to £5,400£3,000 to £4,500 ex VAT

Site set-up: welfare, protection, hoarding, deliveries	1 item		£2,500 to £4,000£2,100 to £3,300 ex VAT
Skips and waste removal	3 nr	£380 to £460 each	£1,100 to £1,400£1,000 to £1,200 ex VAT

Demolition and groundworks20%£10,800 to £16,800£9,000 to £14,000 ex VAT

Demolition and strip-out of the rear wall, openings and garden	20 m²	£80 to £130/m²	£1,600 to £2,600£1,300 to £2,200 ex VAT
Strip foundations, London clay depth, including trench and concrete	13 m	£320 to £480/m	£4,200 to £6,200£3,500 to £5,200 ex VAT
Oversite, insulated concrete floor slab and damp-proofing	20 m²	£140 to £200/m²	£2,800 to £4,000£2,300 to £3,300 ex VAT
Below-ground drainage, new runs, gullies and connection	1 item		£2,200 to £4,000£1,800 to £3,300 ex VAT

Structure26%£14,100 to £21,300£11,800 to £17,800 ex VAT

Cavity external walls in brick and block, with insulationPer m² of wall.	39 m²	£260 to £360/m²	£10,100 to £14,000£8,500 to £11,700 ex VAT
Structural steel beams, padstones and installationOne over the new rear opening and one across the bifold span. The knock-through steel is priced in the extras.	2 nr	£1,800 to £3,200 each	£3,600 to £6,400£3,000 to £5,300 ex VAT
Lintels, wall ties and cavity closers	1 item		£400 to £900£300 to £800 ex VAT

Roof14%£7,600 to £11,900£6,300 to £9,900 ex VAT

Warm-deck flat roof: insulation, GRP or EPDM covering, trims	21 m²	£190 to £270/m²	£4,000 to £5,700£3,300 to £4,700 ex VAT
Rooflights, supplied and fitted	2 nr	£1,500 to £2,600 each	£3,000 to £5,200£2,500 to £4,300 ex VAT
Fascias, soffits, gutters and downpipes	13 m	£50 to £80/m	£600 to £1,000£500 to £900 ex VAT

Windows and doorsThe bifold or sliding doors are priced with the fit-out, under the cost.1%£600 to £1,100£500 to £900 ex VAT

Side window, obscure-glazed where near a boundary

1 nr

£600 to £1,100 each

£600 to £1,100£500 to £900 ex VAT

First fix6%£3,300 to £5,600£2,800 to £4,700 ex VAT

Electrical first fix: circuits, cabling, consumer unit changes	20 m²	£60 to £90/m²	£1,100 to £1,700£900 to £1,400 ex VAT
Plumbing and heating: radiators or underfloor heating, pipework	20 m²	£70 to £130/m²	£1,400 to £2,600£1,200 to £2,200 ex VAT
Insulation to walls, floor and roof to Building Regulations	20 m²	£40 to £70/m²	£800 to £1,300£700 to £1,100 ex VAT

Finishes and second fix10%£5,000 to £8,600£4,100 to £7,200 ex VAT

Plasterboard, skim and ceilingsPer m² of floor.	20 m²	£90 to £130/m²	£1,800 to £2,600£1,500 to £2,200 ex VAT
Floor finish: engineered timber, tiles or similar	20 m²	£60 to £140/m²	£1,200 to £2,800£1,000 to £2,300 ex VAT
Skirting, architraves, internal doors	18 m	£30 to £50/m	£500 to £800£400 to £700 ex VAT
Electrical second fix: sockets, switches, lighting	20 m²	£30 to £50/m²	£600 to £1,000£500 to £800 ex VAT
Decoration throughout	20 m²	£50 to £70/m²	£900 to £1,400£800 to £1,200 ex VAT

External works4%£1,500 to £3,500£1,300 to £2,900 ex VAT

Making good outside: paving, steps, external drainage, garden reinstatement

1 item

£1,500 to £3,500£1,300 to £2,900 ex VAT

Builder's preliminaries and overheads12%£5,600 to £11,100£4,600 to £9,300 ex VAT

Builder's preliminaries, supervision and overheads

12% to 15% of the trades above.

£5,600 to £11,100£4,600 to £9,300 ex VAT

Building work total

£52,000 to £85,500£43,500 to £71,000 ex VAT

New external walls about 13 m long and 3 m high; warm-deck flat roof.
Foundations assumed to be conventional strip footings; trees, drains or poor ground change this.

Cross-check: published London guides put a single-storey rear extension of this size at £68,000 to £84,000 (£3,400 to £4,200 per m²). The itemised figures above are the estimate; the guide is a sanity check.

Planning and permissions

Larger home extension: prior approval needed.

Larger home extension: prior approval needed. Class A, paragraph A.1(g) with the A.4 prior approval procedure: 3 to 6 m on an attached house, neighbour consultation of 21 days, 42-day determination, deemed approval on silence. The height limits (4 m overall, 3 m eaves within 2 m of a boundary) and the 50% curtilage test still apply, as does the materials condition. Not available in a conservation area, which this is not. A lawful development certificate on completion is worth advising for the sale file.

✓ passConservation area. The planning register shows no conservation area at this postcode. (planning.data.gov.uk conservation-area dataset)
✓ passArticle 4 direction. The only Article 4 direction recorded here (Article 4 for HMOs) concerns a different kind of development, not householder extensions or lofts. (planning.data.gov.uk)
? checkRear projection. 4 m is beyond the standard 3 m but within the 6 m larger home extension allowance. That route needs prior approval from the council, with neighbours consulted, and is not available in a conservation area. (GPDO 2015 Sch. 2 Pt 1 A.1(g), A.4)
? checkHeight. Single storey: no higher than 4 m, and eaves no higher than 3 m if within 2 m of a boundary. A 3 m flat roof at the boundary is the usual answer on a terrace. (GPDO 2015 Sch. 2 Pt 1 A.1(f), A.1(i))
? checkHalf the garden. Extensions and outbuildings together may not cover more than half the land around the original house. (GPDO 2015 Sch. 2 Pt 1 A.1(b))
? checkMaterials. Materials must be of similar appearance to the existing house, except for a conservatory. (GPDO 2015 Sch. 2 Pt 1 A.3(a))

What else applies

Party Wall etc. Act 1996 applies
Building up to or astride the boundary needs a line of junction notice (1 month). Digging foundations within 3 m of a neighbour's structure, and deeper than theirs, needs a notice of adjacent excavation (1 month). Each neighbour can consent or dissent; dissent means surveyors, which you pay for.
Nothing if neighbours consent in writing within 14 days. Silence counts as dissent. One agreed surveyor runs £700 to £1,800 plus VAT; one each, £1,500 to £3,500 in total. You pay.
Building Regulations approval applies
Every extension and loft conversion needs building control sign-off: foundations, structure, fire escape, insulation, drainage, electrics. Use the council or a registered building control approver. Ask for the completion certificate at the end; you will need it to sell.
Structural engineer applies
Steels for the opening into the house, the roof and any glazing spans need calculations.
Thames Water build-over agreement check
If a public sewer runs within 3 m of the new foundations, Thames Water must agree the design first. Many London terraces have a shared sewer at the rear. Your designer checks the sewer map early.
Thames Water's fee is £130 for a small pipe or £490 plus a CCTV survey for a larger one; the extra foundation work is the real cost. Allow 3 weeks.
Community Infrastructure Levy does not apply
Householder extensions under 100 m² that do not create a new dwelling are exempt, and larger ones can claim the self-build exemption, but only if the forms are filed before work starts.

Timeline

About 23 to 43 weeks from today to moving back in, of which 8 to 16 weeks on site. Builders are often booked 3 to 6 months ahead, which can add to this.

Solid lines are the shorter estimate, dashed the longer. The build on site is in red.

01Design and drawings4 to 8 wkMeasured survey, sketch options, the drawings for planning or the lawful development certificate.
02Prior approval6 to 8 wkThe council consults neighbours for 21 days and must decide within 42 days, or approval is deemed.
03Structural design, building regulations drawings, tender4 to 8 wkEngineer's calculations, the building regulations package, and getting three like-for-like quotes.
04Party wall notices4 to 10 wkRuns alongside tender. One month for line of junction or excavation notices, two months for a party structure notice, longer if a neighbour dissents.
05Build8 to 16 wkFoundations, drainage, walls, roof, glazing, first fix, plaster, second fix, kitchen or bathroom, decoration.
06Completion and sign-off1 to 3 wkSnagging, building control's final inspection, the completion certificate and the party wall award closed.

Your options

inc VAT

What moves the price

Each line re-prices the same house with one thing changed. Use it to see what a decision is worth before you make it.

What else this house could take

Job	Typical size	Likely range	Route
Side-return extension	10.8 m²	£46,000 to £75,000	Larger home extension: prior approval needed
Loft conversion with rooflights only	20 m²	£56,000 to £89,000	Likely permitted development (no planning application)
Loft conversion with a rear dormer	28 m²	£73,000 to £113,000	Likely permitted development (no planning application)
Wraparound (side return and rear)	26 m²	£78,000 to £120,000	Larger home extension: prior approval needed
L-shaped dormer loft conversion	36 m²	£87,000 to £134,000	Likely permitted development (no planning application)
Double-storey rear extension	33 m²	£95,000 to £147,000	Likely permitted development (no planning application)
Mansard loft conversion	40 m²	£116,000 to £174,000	A householder planning application is needed

Each is priced for this house at a typical size with the same finish and no extras (an en suite for lofts), so the figures are for comparing options, not for planning one.

Before you get quotes

Risks for this job

Prior approval turns on neighbour amenity. A 3 m fallback on the drawings keeps the kitchen layout viable if an objection is upheld.
Soft Victorian brickwork at the rear: allow for a goalpost frame rather than padstones alone when the engineer surveys.
Shared sewer at the rear is common on this street type; check the Thames Water asset map before fixing the foundation line.
Two party structure notices; a single dissent adds £1,500 to £3,000 and four to eight weeks to the programme.
A 5 m bifold span needs a stiff lintel and a glazing specification fixed before tender, or the provisional sum will drift.
Contractor lead times in NW London are three to six months; tender early against the building regulations package.

Questions to ask anyone who quotes

Is the tender a fixed price against the drawings and specification issued, and what is excluded?
Are figures ex VAT, and is the contractor VAT registered?
Which items are provisional sums, and what quantities are they based on?
What is the approach if drains are found within 3 m of the foundations?
Is the structural steel package priced on the engineer's drawings or on an allowance?
What is the payment schedule and is it tied to stages?
Who is the site manager and how many concurrent jobs are they running?
Two references for comparable rear extensions completed in the last twelve months.

Next steps

Measured survey and two options: 4 m with prior approval, 3 m within Class A.
Check the Thames Water asset map and the council's conservation area map at the plot.
Prior approval notification; structural design and building regulations package in parallel.
Party wall notices on both sides once the structural drawings are fixed.
Tender to three contractors on an identical package; compare provisional sums line by line.
Advise the client to ring-fence the contingency.