House Extensions Forest Hill
Single storey rear and double storey extensions in Forest Hill, planning, structure and build under one contract.
- Rated 4.9 from 31 reviews on MyBuilder
- Single-storey rear extension from £40,000
- Free site survey, itemised quote
Across Forest Hill, GS Renovation extends bay-fronted Victorian and Edwardian terraces with rear, side-return and wrap-around builds. We handle the foundations, steel beams, roof works and fit-out as one project, and we tell you honestly whether your extension is permitted development or needs a planning application to Lewisham Council.
In Forest Hill, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative London ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Forest Hill: the structural engineer we appoint designs the structure, we build it, and building control signs it off.
Price guide
| Work | Indicative London range |
|---|---|
| Single-storey rear extension | from £40,000 |
| Rear extension, per m² | £2,000 to £2,800 |
| Side return, per m² | £2,400 to £3,200 |
| Wraparound, per m² | £2,400 to £3,200 |
| Double storey, per m² | £1,800 to £2,600 |
| Basement extension | see the basement page |
Indicative London ranges (Architecture for London, Resi, MyBuilder and Checkatrade, 2026), not a quotation. Prices checked September 2026. Full breakdown: Extension Costs & Planning.
Quick estimate for Forest Hill
Forest Hill range: choose above
Side-return extensions in Forest Hill
The narrow side return running past the closet wing is the classic space to claim in an SE23 terrace. By roofing over that alley and knocking through, we widen a cramped galley kitchen into a bright open-plan kitchen-diner. This means new foundations next to the neighbour's boundary, a steel beam along the old external wall, and often rooflights to pull daylight deep into the plan. The engineer sizes the steel and building control inspects each stage.
Many side-return and modest rear extensions on these terraces fall within permitted development, but that is never guaranteed, and a home in a conservation area or one that has already been extended may need full planning permission from Lewisham Council. We check the constraints first, arrange the drawings, and plan the programme around the party wall notices you serve as building owner, which nearly always apply where you build up to a shared boundary on these SE23 streets.
Housing in Forest Hill
Around 11,040 homes make up Forest Hill on the valuation list; 58% are flats and 42% houses, and 54% date from before 1919, 13% from between the wars and 30% from after 1945. A flat in Forest Hill has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent, which matters here because most homes are flats.
Getting a planning officer's view on a Forest Hill extension early
Written advice from a planning officer is available before you commit to a design. Lewisham's Duty Planner Standard Service gives written advice on simple homeowner projects, and it covers simple householder extensions and alterations not within a conservation area. For a Forest Hill house outside the conservation area, that is a way to test the depth, height and roof form of a rear or side extension before paying for a full set of drawings. Within the conservation area that service does not cover extensions, so the advice route there is different.
Inside the Forest Hill conservation area, a side extension or a two storey rear extension needs planning permission, while a single storey rear extension can still be permitted development unless an Article 4 direction removes it. Any application there needs a heritage statement, because Lewisham requires one for all planning applications affecting buildings in conservation areas. On either side of the boundary, the Lewisham Local Plan says extensions to residential buildings should normally be subordinate in scale to the original building. On a villa with a gap at the side, we read that as an extension set back from the front and kept below the main roof, and we draw the first scheme that way.
Double-storey extensions on Forest Hill hills
On the leafy hills of SE23, a double-storey rear extension adds a larger kitchen below and an extra bedroom or en-suite above, working with the slope many Forest Hill plots sit on. We dig and pour foundations to suit the ground and any nearby trees, build the shell to match the existing brick and bay detailing, and tie the new roof neatly into the original. Steel beams carry the openings between old and new.
Because the extra storey rises alongside the neighbouring terrace, both the Party Wall Act and, usually, a full planning application to Lewisham Council come into play, and we guide you through both. Building regulations then govern the structure, insulation, fire escape and drainage. From groundworks to the final coat of paint, one GS Renovation team runs the programme, keeping the rest of your home liveable while the extension goes up.
Kitchen extensions in Forest Hill
58% of homes in Forest Hill are flats, and a flat has no householder permitted development, so a ground-floor flat's kitchen extension needs planning permission from Lewisham Council and the freeholder's consent. Most kitchen extensions here are on the 42% of homes that are houses. The fit-out is priced on its own page, kitchen renovation in Forest Hill; the build figures are in the table above.
- Planning applications and permitted development assessments
- Building control inspections at foundation, damp course and steel stages
- Full fit-out, finishing and making good
VOA Council Tax stock of properties, March 2025, for Forest Hill West, Forest Hill East and Honor Oak. Counts rounded by the VOA.
Why Forest Hill Homeowners Choose GS Renovation
Homeowners across Forest Hill choose GS Renovation because we build extensions that suit these specific terraces, not generic boxes. We are straight with you about permitted development versus a Lewisham planning application, we give you the drawings and programme your party wall notices need, and we self-deliver the groundworks, steelwork, roofing and finishes. Our founder's 30+ years on the tools stand behind every project.
- Waste cleared as we go, not left piled up outside
- Insulation and thermal detailing to current regulations
- Foundation depth set after trial holes, drains and trees are checked
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
George was brilliant. Lovely bloke, very reasonably priced and a perfectionist to boot. He completed our small kitchen renovation with care and great…
Giorgio is an excellent and intelligent workman, and with very reasonable prices! He installed a kitchen extractor fan in a complicated situation…
FAQ
Frequently Asked Questions about House Extensions Forest Hill
Do you charge for a survey before quoting house extensions in Forest Hill?
GS Renovation charges nothing to survey an SE23 property for house extensions, because pricing house extensions in Forest Hill unseen is guesswork. The visit confirms the scope, what approvals it needs (building regulations approval from foundations to completion), and where in £2,000 to £2,800 per m² for a rear extension (indicative) the job sits. An itemised figure follows in writing. Call 07472 424 226 or email info@gsrenovationltd.co.uk.
Will my Forest Hill side-return extension need planning permission?
Often it will not, as many side-return and single-storey rear extensions in Forest Hill qualify as permitted development. However, if your terrace sits in a conservation area, is already extended, or you plan to go higher or wrap around, a planning application to Lewisham Council may be required. We assess your SE23 property and existing consents before drawing anything, so you know the route from the start.
How deep will extension foundations go on Forest Hill clay?
Deeper than the minimum where trees are close, because London clay shrinks as roots draw moisture out. Building control confirms the depth on site once the trench is open, taking account of nearby trees, and an engineer may specify piles or a reinforced design where the ground is poor. On sloping SE23 plots the trench can also step down the hill, which changes the brickwork below ground level.
What if a drain runs under my planned Forest Hill extension?
Locate and map it before anything is drawn. Building over or close to a public sewer requires the water company to agree first, and their terms can alter the footings or oblige you to divert the pipe. Where the run is private and serves only your house, it can generally be redirected or spanned with lintels, with an inspection chamber kept for access. A camera survey settles which case applies.
Must I notify my Forest Hill neighbours before digging extension foundations?
Yes, where the trench goes deeper than their foundations within 3 metres of their home: the Party Wall Act then asks for a month's notice. A new wall astride or on the boundary line also needs notice. Each neighbour may consent or dissent, and a dissent ends in surveyors drawing up an award, so serve early. On terraced SE23 streets that covers almost every rear extension.
Which building control inspections happen on a Forest Hill extension?
Typically the foundation trench before concrete, the damp proof course and oversite, drains before backfilling, the steel where the house is opened into, and a final visit before the completion certificate. Part L requires insulation in the floor, walls and roof to meet current standards, so the inspector will want to see it before it is covered. Lewisham Council can run these stages.
Which building control route suits a Forest Hill extension within 3 metres of a sewer?
Expect a full plans application rather than a building notice. Lewisham uses a full plans application for any new proposal to build above or within 3 metres of a drain or sewer, and it sends its decision within five weeks unless you have given a three week extension. Allow for that in the programme, and have the sewer located before the drawings go in, since its position can move the footings.
Does adding a porch to a Forest Hill house need planning permission?
It can, even for a small one. Lewisham lists the addition of a porch, including infilling an open porch, among the changes needing planning permission under an Article 4 direction. Those directions only apply to houses in use as a single dwelling house. So a family house on a Forest Hill street covered by a direction needs an application before an open porch is glazed in, and we check for a direction at the address before drawing it.
How should a Forest Hill extension that also adds a WC be described to building control?
As the whole scope, from the first application. Lewisham's building control fees chart separates a single storey extension with no alterations from one that includes a new toilet or bathroom or removal of a chimney breast. Lewisham also says small domestic projects such as extensions fall under schedule 1 of the chart in most cases. Naming the cloakroom WC at the start means the charge and the inspections match the work actually being built.
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