Basement Conversions · SE23 Forest Hill
SE23 is decided by Lewisham Council. Forest Hill and Honor Oak are hilly, leafy Victorian/Edwardian suburbs with strong loft and rear-extension demand; conservation areas cover the better villa streets near the Horniman and London Road, while the bulk of the terraced and semi stock keeps permitted-development rights.
If your property sits elsewhere in the borough, the borough-wide page for basement conversions in Lewisham covers the same service across every ward.
Basement conversions and lowering existing cellars in Forest Hill add space without touching the roofline — valuable on SE23's period houses around Forest Hill, Honor Oak, Perry Vale, Horniman. Lewisham Council reviews structural, drainage and flood-risk impact, and a Party Wall award plus a Thames Water build-over agreement are usually required. We coordinate the structural engineer and the waterproofing design.
Full architectural technology service for basement digs and cellar conversions in Forest Hill — structural underpinning design, waterproofing strategy, Party Wall coordination, planning where required, and building regulations through to completion certificate.
Across Forest Hill, Honor Oak, Perry Vale, Horniman in SE23, the dominant property is victorian and edwardian terraces and villas across forest hill and honor oak, larger detached houses on the hills, and 1930s semis towards perry vale. That stock shapes what basement conversion drawings looks like here: the right approach for a period terrace differs from a mansion-block flat or a new-build, and Lewisham Council judges each on design quality and neighbour impact. We survey the specific Forest Hill property, confirm the planning route — Permitted Development, a Lawful Development Certificate, or full planning — and draw a council-ready set tailored to the SE23 streetscape.
Housing stock in SE23: Victorian and Edwardian terraces and villas across Forest Hill and Honor Oak, larger detached houses on the hills, and 1930s semis towards Perry Vale.
Within the district: Forest Hill, Honor Oak, Perry Vale, Horniman.
Conservation areas: Forest Hill, Honor Oak, Horniman / London Road. Inside these, most external work needs full planning and a Heritage Statement.
Getting around SE23: Forest Hill and Honor Oak Park (Overground and Southern).
Property values: approx £600k–£1.2m for houses, £375k–£600k for flats (Land Registry, 2024) — a useful gauge of the build value an extension or loft adds in SE23.
Borough character: Generally permissive planning compared to central boroughs; loft conversions and side-return extensions routinely approved under PD.
Desktop study plus measured survey assessing existing foundations, ground conditions, water table level and underpinning viability for your Forest Hill property.
New-build basements and lightwell extensions need planning permission. Cellar conversions of existing space are usually Permitted Development. We confirm the route and submit where needed.
Underpinning sequence drawings, reinforced concrete design, temporary works strategy — all to Eurocode with chartered engineer sign-off.
BS 8102:2022 compliant — cavity drain, cementitious tanking or membrane system specified for Forest Hill's ground conditions and water table.
Full Plans application to Lewisham Council Building Control or an approved inspector covering structure (Part A), fire safety (Part B), damp-proofing (Part C), ventilation (Part F) and thermal performance (Part L).
Almost every Forest Hill basement triggers Party Wall notices. We also manage Thames Water Build Over Agreements where within 3m of a public sewer.
Single planning or building-regs submission.
Planning + building regs + structural.
Listed, conservation or complex sites.
Most do. The Victorian and Edwardian terraces and semis across Forest Hill and Honor Oak in SE23 generally keep permitted-development rights for single-storey rear extensions and rear dormer lofts, decided by Lewisham Council, provided they're outside the local conservation areas. The hilly plots sometimes raise overlooking and retaining-wall issues we design around. We check your address against Lewisham's conservation layer first.
Parts are. Lewisham designates conservation areas in Forest Hill and Honor Oak, typically covering the grander villa streets near the Horniman Museum and London Road, while most of the terraced and 1930s semi stock sits outside. Inside a conservation area, loft dormers and external alterations need full planning and a Heritage Statement. We map your specific Forest Hill or Honor Oak street before relying on PD.
Forest Hill and Honor Oak SE23 have a lot of larger Victorian and Edwardian semi-detached and detached villas, which often take a hip-to-gable plus rear dormer for a full extra floor, or a substantial rear dormer on terraces. The sloping sites mean we pay attention to ridge heights and neighbour overlooking. We design to the roof type and confirm whether full planning or a Lawful Development Certificate applies.
With Forest Hill property at approx £600k–£1.2m for houses, £375k–£600k for flats (Land Registry, 2024), the professional drawing fee is a small fraction of the value the work adds to SE23's victorian and edwardian terraces and villas across. Lewisham Council charges its own statutory application fee on top (currently £548 for a householder planning application, £610 for a full application). We quote the drawings as one fixed price and the free site survey covers every SE23 address.
For a SE23 project across Forest Hill, Honor Oak, Perry Vale, Horniman, expect roughly 3–4 weeks from instruction to submission: week 1 the measured survey, weeks 2–3 the drawings and your revisions, then validation and the statutory determination by Lewisham Council (8 weeks for a householder application). We carry the Forest Hill application through every officer query to the decision.
Forest Hill (SE23) sits primarily in Lewisham, so applications are decided by Lewisham Council. Conservation areas within SE23 include Forest Hill, Honor Oak, Horniman / London Road, where full planning and a Heritage Statement are required. We confirm the planning route for your exact address before quoting.
Almost always. Any structural work behind basement conversion drawings in Forest Hill — new openings, steel beams, loft floors, foundations — needs Building Regulations approval covering structure (Part A), fire escape (Part B) and insulation (Part L), submitted to Lewisham Council Building Control or an approved inspector. On SE23's typically terraced and semi-detached stock, a Party Wall notice to adjoining neighbours is normally required before the steelwork. We coordinate both as part of the Complete package.
In value terms, yes. With Forest Hill (SE23) property at approx £600k, £1.2m for houses, £375k, £600k for flats (Land Registry, 2024), a well-designed extension or loft typically returns more than it costs in added floor area and sale value. The bigger risk is an unapproved or non-compliant scheme — which is why council-ready drawings and a clean Building Control sign-off matter on SE23 homes.
Moving, or weighing up a site in the next borough along? Here is the complete service list for SE3, SE4, SE6, SE12, SE13 and SE26.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
Every SE23 postcode district we draw in, the stock that dominates it and the authority that validates a basement conversion there. SE23 fees below are the statutory England rates for 2026, not basement conversion estimates.
| Postcode | Area | Dominant stock | What it means for a basement conversion |
|---|---|---|---|
| SE23 | Forest Hill / Honor Oak | Victorian villas, Edwardian terraces | Forest Hill / Honor Oak sits wholly inside SE23, so Lewisham Council validates every basement conversion submitted from an SE23 address. |
| SE23 | Forest Hill | Victorian villas, Edwardian terraces | Forest Hill sits inside SE23; basement conversion proposals here are determined by Lewisham Council. |
| SE23 | Honor Oak | Victorian villas, Edwardian terraces | Honor Oak sits inside SE23; basement conversion proposals here are determined by Lewisham Council. |
| SE4 | Brockley | Victorian and Edwardian terraces | Adjoining district to SE23, also inside Lewisham for basement conversion purposes. |
| SE6 | Catford / Bellingham | 1930s semi-detached, post-war flats | Adjoining district to SE23, also inside Lewisham for basement conversion purposes. |
| SE8 | Deptford | Georgian terraces, post-war estates, new-build | Adjoining district to SE23, also inside Lewisham for basement conversion purposes. |
| SE12 | Lee / Grove Park | mixed stock | Adjoining district to SE23, also inside Lewisham for basement conversion purposes. |
Roof pitch, wall build-up and plot depth change every few SE23 streets, and each one changes what a basement conversion can achieve at that address.
SE23 (Forest Hill / Honor Oak) conservation designations: there are 30 across Lewisham Council, Lee Green, Blackheath, Deptford High Street among them. Designation costs a SE23 (Forest Hill / Honor Oak) owner the permitted-development route: a basement conversion then needs a £548 householder application over 8 weeks, in place of the £249 prior approval route that takes 6 weeks. Deptford High Street is the SE23 (Forest Hill / Honor Oak) designation that most often changes a basement conversion design.
SE23 (Forest Hill / Honor Oak) permitted development status: there is no borough-wide direction from Lewisham Council, so a basement conversion at a SE23 address is normally still permitted development. We certify that with a £274 Lawful Development Certificate rather than leaving a SE23 (Forest Hill / Honor Oak) sale to argue it later.
SE23 (Forest Hill / Honor Oak) housing stock: Victorian and Edwardian terraces across Brockley, Honor Oak, Forest Hill; 1930s semi-detached in Catford and Hither Green; post-war mixed in Deptford. Generally permissive planning compared to central boroughs; loft conversions and side-return extensions routinely approved under PD. Read that before choosing a basement conversion route in SE23 (Forest Hill / Honor Oak) — construction, not postcode, sets what is achievable. Lewisham Council charges £548 for a householder application and has 8 weeks to determine it.
Sources for this SE23 basement conversion page: borough policies maps and conservation-area registers (2025–26), Royal Mail postcode district boundaries, the Town and Country Planning (Fees for Applications) (England) Regulations, the Town and Country Planning (General Permitted Development) (England) Order 2015 as amended, and the Party Wall etc. Act 1996.
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Local values run approx £600,000–£1.2m for houses, £375,000–£600,000 for flats. Source: Land Registry, 2024.
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