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
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.
- 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.
Expect a 20 m² single-storey rear extension with a new kitchen and bifold doors to cost about £120,500 all in, most likely somewhere between £95,500 to £146,000. That covers the building work, the kitchen and doors, and the professional and council fees; with the contingency you should hold back on top, have £154,500 available. A rear extension of up to 3 m on a terrace is usually permitted development; yours is 4 m, which puts it in the prior approval route, where the council asks the neighbours before it can go ahead. Two party walls are affected, so budget for party wall surveyors unless both neighbours consent in writing. From first sketch to moving back in, allow 23 to 43 weeks.
The house
The EPC register has this as a 1900 to 1929 mid-terrace house of about 94 m² with no extension recorded, which is what you told us and what the street suggests: a Victorian or Edwardian terrace with a back addition. The planning register shows no conservation area at this postcode and the only Article 4 direction recorded in Brent concerns houses in multiple occupation, not extensions, so it does not touch you. The flood zone is 1, the lowest. None of that is a survey of your house: it tells us the rules that apply, not the state of the drains or the footings.
- 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
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 VATMost of the money goes into the shell: the foundations and slab on London clay, the new walls with a long steel to open up the back of the house, and the roof and glazing. The kitchen, at £16,000 to £28,000, is the biggest single choice you have made, and the party wall line only becomes real if a neighbour dissents. What would push this job towards the top of its range is the ground: drains that are not where the drawing says, or footings that need to go deeper. The choice worth most is the depth: one metre shallower, at 3 m, would bring the expected cost down by £10,500 and take you back inside permitted development.
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 |
Fit-out you chose
| New kitchen: units, worktops, appliances, fitting | £16,000 to £28,000 |
| Bifold or sliding doors | £7,000 to £11,000 |
| Knocking through: steel beam, making good | £4,500 to £8,000 |
Fees and approvals
| Architect or designer: drawings, planning and building regulations package | £4,200 to £11,100 |
| Structural engineer: calculations and beam design | £1,000 to £2,500 |
| Council prior approval feeStatutory fee plus the Planning Portal's service charge. | £334 |
| Building control: plan check and inspections | £1,000 to £1,800 |
| 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 |
| Thames Water build-over agreement (if a public sewer is within 3 m) | £0 to £1,200 |
| Expected costLikely £95,500 to £146,000 | £120,500 |
| 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 |
| Budget to have availableThe top of the likely range plus the contingency. | £154,500 |
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 include VAT at 20%. Builders often quote without VAT, so add 20% to any quote that says 'plus VAT' before comparing it with this report.
Where the money goes
inc VATThe 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
| Site set-up: welfare, protection, hoarding, deliveries | 1 item | £2,500 to £4,000 | |
| Skips and waste removal | 3 nr | £380 to £460 each | £1,100 to £1,400 |
Demolition and groundworks20%£10,800 to £16,800
| Demolition and strip-out of the rear wall, openings and garden | 20 m² | £80 to £130/m² | £1,600 to £2,600 |
| Strip foundations, London clay depth, including trench and concrete | 13 m | £320 to £480/m | £4,200 to £6,200 |
| Oversite, insulated concrete floor slab and damp-proofing | 20 m² | £140 to £200/m² | £2,800 to £4,000 |
| Below-ground drainage, new runs, gullies and connection | 1 item | £2,200 to £4,000 |
Structure26%£14,100 to £21,300
| Cavity external walls in brick and block, with insulationPer m² of wall. | 39 m² | £260 to £360/m² | £10,100 to £14,000 |
| 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 |
| Lintels, wall ties and cavity closers | 1 item | £400 to £900 |
Roof14%£7,600 to £11,900
| Warm-deck flat roof: insulation, GRP or EPDM covering, trims | 21 m² | £190 to £270/m² | £4,000 to £5,700 |
| Rooflights, supplied and fitted | 2 nr | £1,500 to £2,600 each | £3,000 to £5,200 |
| Fascias, soffits, gutters and downpipes | 13 m | £50 to £80/m | £600 to £1,000 |
Windows and doorsThe bifold or sliding doors are priced with the fit-out, under the cost.1%£600 to £1,100
| Side window, obscure-glazed where near a boundary | 1 nr | £600 to £1,100 each | £600 to £1,100 |
First fix6%£3,300 to £5,600
| Electrical first fix: circuits, cabling, consumer unit changes | 20 m² | £60 to £90/m² | £1,100 to £1,700 |
| Plumbing and heating: radiators or underfloor heating, pipework | 20 m² | £70 to £130/m² | £1,400 to £2,600 |
| Insulation to walls, floor and roof to Building Regulations | 20 m² | £40 to £70/m² | £800 to £1,300 |
Finishes and second fix10%£5,000 to £8,600
| Plasterboard, skim and ceilingsPer m² of floor. | 20 m² | £90 to £130/m² | £1,800 to £2,600 |
| Floor finish: engineered timber, tiles or similar | 20 m² | £60 to £140/m² | £1,200 to £2,800 |
| Skirting, architraves, internal doors | 18 m | £30 to £50/m | £500 to £800 |
| Electrical second fix: sockets, switches, lighting | 20 m² | £30 to £50/m² | £600 to £1,000 |
| Decoration throughout | 20 m² | £50 to £70/m² | £900 to £1,400 |
External works4%£1,500 to £3,500
| Making good outside: paving, steps, external drainage, garden reinstatement | 1 item | £1,500 to £3,500 |
Builder's preliminaries and overheads12%£5,600 to £11,100
| Builder's preliminaries, supervision and overheads | 12% to 15% of the trades above. | £5,600 to £11,100 |
| Building work total | £52,000 to £85,500 |
- 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. The standard permitted development limit for a rear extension on an attached house is 3 m. Between 3 m and 6 m you can use the larger home extension route: you tell the council before you start, it writes to the neighbours, and if nobody objects within 21 days it is deemed approved after 42 days. If a neighbour objects, the council decides whether the effect on their amenity is acceptable. The height rules still apply: no higher than 4 m, and eaves no higher than 3 m within 2 m of the boundary, which on a terrace means a flat roof at about 3 m at the fence. After the work, a lawful development certificate is worth having for when you sell.
- ✓ 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.
- 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 VATWhat 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
- The 4 m depth relies on prior approval. If either neighbour objects and the council sides with them, the fallback is 3 m, which changes the kitchen layout. Design for both from the start.
- A long steel across the back of a Victorian terrace needs padstones and often a goalpost frame. If the engineer finds the existing brickwork is soft, the cost of that line rises.
- Drains on terraces of this age often run across the back garden. If a shared sewer is within 3 m of the new foundations, Thames Water must agree the build-over before you dig.
- Two party walls means two sets of notices. One dissenting neighbour adds about £1,500 to £3,000 and four to eight weeks.
- Bifold doors across a 5 m opening are heavy and need a stiff lintel; cheap frames warp. The glazing specification is the single biggest quality decision in this job.
- Builders in north-west London are booking three to six months ahead. The timeline assumes you can start when the paperwork is ready.
Questions to ask anyone who quotes
- Is your price fixed, or an estimate subject to measurement, and what is excluded from it?
- Does the price include VAT, and are you VAT registered?
- Who is the structural engineer, and is their design included or extra?
- Which items are provisional sums, and what are they for?
- How will you handle the drains if they are not where the drawing shows?
- What is your payment schedule, and is it tied to stages of work rather than dates?
- Who is on site day to day, and how many other jobs will you be running?
- Can I speak to two customers whose rear extensions you finished in the last year?
Next steps
- Get a measured survey and sketch options from an architect or designer, including a 3 m version.
- Ask the designer to confirm the roof height and eaves at the boundary against the permitted development rules.
- Submit the prior approval notification and, in parallel, get the structural design and building regulations drawings done.
- Serve party wall notices on both neighbours as soon as the drawings are fixed.
- Send the same drawings and specification to three builders and compare like for like.
- Hold the contingency in a separate account and do not touch it for upgrades.