Sitting within the SE23 postcode in the London Borough of Lewisham, Forest Hill has steadily grown into one of South East London's most appealing residential destinations for families and long-term homeowners. From the handsome Victorian terraces lining Thorpewood Avenue and Devonshire Road, to the larger Edwardian semis surrounding Horniman Gardens and the well-kept period streets near Forest Hill station, the area combines genuine architectural character with strong community identity and excellent green space on its doorstep.
For SE23 homeowners, extending rather than relocating is the most practical route to gaining the space a growing household needs. Forest Hill's property values have grown consistently, supported by fast rail links into London Bridge and Victoria and an expanding local food and culture scene. A well-executed extension delivers a meaningful improvement to daily life and a sound return at resale. This guide is written specifically for Forest Hill homeowners and draws on Buildaway's experience delivering extensions across Lewisham, combining transparent 2026 pricing, local planning knowledge, and an 18-month labour warranty on every completed build.
How Much Does a House Extension Cost in Forest Hill (2026)?
Forest Hill sits in the mid range of South East London's construction market, offering competitive build costs relative to inner-London postcodes while reflecting the quality and scale of SE23's Victorian and Edwardian housing stock. Below are realistic 2026 mid-market figures:
Single-Storey Rear Extension
- Build-only: £2,400 – £3,200 per m²
- Fully finished with kitchen: £3,200 – £4,400 per m²
- Example: 25 m² kitchen-diner = £60,000 – £80,000 build-only or £80,000 – £110,000 turnkey.
Double-Storey Rear Extension
- Build-only: £2,200 – £3,000 per m²
- Fully finished: £3,000 – £4,200 per m²
- Example: 40 m² across two floors = £88,000 – £120,000 build-only or £120,000 – £168,000 turnkey.
Side Return & Wrap-Around Extensions
A well-suited option for Forest Hill's Victorian mid-terrace stock: £2,400 – £3,600 per m², depending on structural requirements, roof specification, and glazing choice.
All figures reflect fully compliant, Building Control certified builds meeting Lewisham Council's current planning and construction standards.
What Drives the Price Up or Down
- Total floor area — the primary cost variable across all project types; a larger footprint increases structural, insulation, and finishing expenditure in direct proportion.
- Structural steelwork — load-bearing wall removal is a routine requirement across Forest Hill's Victorian and Edwardian terrace stock and typically adds £3,000–£6,500 to the overall project cost.
- Roof design and specification — flat roofs with quality membranes and rooflights remain the most cost-effective solution; pitched or lantern roof designs add cost but can significantly improve natural light in the extended space.
- Glazing and doors — bi-fold or sliding doors opening onto a rear garden are popular across SE23 and add £4,000–£10,000 depending on opening width and frame specification.
- Site levels and topography — Forest Hill's varied terrain means some properties sit on sloped plots, requiring additional groundworks, retaining structures, or stepped foundations that add to the overall build cost.
- Conservation area obligations — parts of Forest Hill, including streets within the Devonshire Road conservation area and surrounding zones, are subject to Lewisham's heritage controls, influencing material choices and design requirements.
- Kitchen and bathroom fit-out — kitchens in the SE23 market typically range from £10,000 to £27,000+; bathrooms from £6,000 to £15,000.
- Heating and mechanical upgrades — Megaflo unvented systems or boiler replacements are commonly required when extensions increase floor area or add new bathrooms: £3,000–£5,500.
- Site access and logistics — residential parking zones, the constraints of Forest Hill's hillside roads, and the density of SE23's housing all contribute to preliminary costs and require careful programme management.
Planning Permission & Building Control in Forest Hill
- Permitted Development (PD): Smaller single-storey rear extensions in Forest Hill may qualify under PD rights, provided they meet Lewisham's criteria for depth, height, and materials. Properties within conservation areas or those subject to Article 4 Directions require careful individual assessment before PD is assumed.
- Neighbour Consultation Scheme: Extensions beyond standard PD depth limits — up to 6 metres for terraced and semi-detached properties, 8 metres for detached — require formal notification to Lewisham Council and neighbouring owners before works begin. Buildaway manages this process on your behalf.
- Full Planning Permission: Required for double-storey extensions, proposals with street-facing visibility, front alterations, and all projects falling outside PD thresholds.
- Conservation Areas: The Devonshire Road conservation area and several other designated zones within Forest Hill impose material and design controls on extensions. Lewisham's planning officers assess all proposals within these areas for visual compatibility with the original building and wider streetscape.
- Party Wall Act: The density of terraced and semi-detached housing throughout Forest Hill means the Party Wall etc. Act 1996 applies to the majority of extension projects in SE23. Written notices must be formally served on all affected adjoining owners before any structural or excavation works commence.
- Building Control: Mandatory for all extension types — covering structural calculations, thermal performance, drainage, fire safety, and electrical compliance. Buildaway coordinates with Lewisham's building control team and accredited private inspectors as appropriate to each project.
How Long Will It Take?
- Design and planning preparation: 4–10 weeks
- Technical drawings, approvals, and statutory notices: 3–7 weeks
- On-site construction:
- Single-storey: 10–14 weeks
- Double-storey: 14–20 weeks
- Conservation area applications, sloped-site groundworks, or bespoke glazing may extend the overall programme by two to four weeks.
Typical Full Project Budget Breakdown (30 m² Double-Storey)
| Element | Typical Cost |
|---|---|
| Core structure & shell | £70,000 – £88,000 |
| Windows / doors / glazing | £5,000 – £10,500 |
| Kitchen supply & fit | £10,000 – £25,000 |
| Bathroom fit-out | £6,000 – £14,000 |
| Heating / boiler / Megaflo | £3,000 – £5,500 |
| Flooring & decorating | £6,000 – £15,000 |
| Professional fees & permissions | £3,000 – £8,500 |
| Total realistic range: | £103,000 – £150,000 |
(Higher-specification finishes, sloped-site groundworks, or conservation area requirements can push costs beyond £180,000.)
Why Forest Hill Clients Choose Buildaway
- Lewisham planning knowledge — practical familiarity with Lewisham Council's planning policies, conservation area guidance, and the Victorian and Edwardian housing stock that defines Forest Hill's residential character.
- Fully transparent pricing — detailed, itemised quotations broken down by trade and project stage, with no vague allowances or charges introduced once work is underway.
- 18-month labour warranty — delivering greater post-completion assurance to Forest Hill homeowners than the standard defects liability period offered by most comparable local contractors.
- Fully accredited and insured — Gas Safe registered, NICEIC certified, and carrying comprehensive public liability insurance on every project without exception.
- Complete project management — Buildaway coordinates architects, structural engineers, Party Wall surveyors, and all specialist trades from first consultation through to final Building Control sign-off.