TL;DR — Key Facts
- Build cost per sqm: £2,500 – £3,800 (inner London borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 30 designated areas in Lewisham
- Permitted Development: Generally available outside conservation areas
Extension costs in Lewisham
Once the budget works, the next step is the drawing set: house extensions in Lewisham.
House extension build costs in Lewisham typically range from £2,500 to £3,800 per square metre, reflecting Lewisham's position as an inner London borough. These figures cover the construction cost only and do not include professional fees, planning fees, or VAT.
The ground rises noticeably across Brockley, Honor Oak and Forest Hill, and on a sloping plot a level rear extension needs retaining and stepped foundations before anything visible is built. That is the single largest variable in this borough. On the flatter Catford and Hither Green stock, the same extension prices as a conventional semi job with a steel and a set of doors.
Typical extension costs in Lewisham (build only)
On top of the build cost, you will need architectural drawings. These fees are 30% below typical London architect rates.
What affects extension costs in Lewisham
Lewisham has the sharpest single number in this family: of the deep rear extensions where prior approval was needed, it granted six and refused 135 (MHCLG planning statistics, retrieved 1 September 2026) - a 4.3% grant rate, the lowest in London. Against that it approves 86.3% of ordinary householder applications and has no borough-wide Article 4 Direction. The cost implication is direct: design to stay inside the permitted limits, not past them.
Conservation areas
Lewisham has 30 designated conservation areas, including Blackheath, Deptford High Street, Forest Hill, Hither Green, Lee Green. If your property is in a conservation area, Permitted Development rights are restricted, meaning you may need a full planning application (£548 council fee). Conservation area applications also tend to require higher-quality materials and more sympathetic design, which can increase build costs by 10-20%.
Property type
The housing stock in Lewisham is predominantly victorian and edwardian terraces across brockley, honor oak, forest hill; 1930s semi-detached in catford and hither green; post-war mixed in deptford. Victorian and Edwardian terraces typically suit side-return and rear extensions, while 1930s semi-detached properties are well suited to two-storey rear extensions. Detached houses offer the most flexibility but often have larger footprints, meaning higher absolute costs even if the per-sqm rate is similar.
Access and logistics
Brockley, Honor Oak and Forest Hill are continuous Victorian and Edwardian terrace, so the rear is reached through the house. Catford and Hither Green's interwar semis have side passages and price differently. The ground rises noticeably across parts of the borough, and on a sloping plot the retaining and levelling work becomes a line item of its own before any superstructure.
Our drawing fees for Lewisham extensions
At Architectural Drawings London. No hourly billing, no surprises. Our fees cover everything needed for a valid planning submission to Lewisham Council and for building regulations approval.
Drawing fees for extensions in Lewisham
Every package includes a measured survey of your existing property, existing floor plans and elevations, proposed floor plans and elevations, a site plan, and a design and access statement where required. Our MCIAT chartered architectural technologists handle the full process from survey to submission, and we liaise with Lewisham Council on your behalf.
We cover all Lewisham postcodes: SE3, SE4, SE6, SE8, SE12, SE13, SE14, SE23, SE26.
Planning vs Permitted Development in Lewisham
Many house extensions in Lewisham can be built under Permitted Development (PD) rights without needing a planning application. Under PD, single-storey rear extensions can extend up to 6 metres from the original rear wall for terraced and semi-detached houses, or 8 metres for detached houses (under the Prior Approval process).
Lewisham's 30 conservation areas stand alone, unaccompanied by any borough-wide Direction, and that makes the designation boundary unusually decisive - national rules on one side, a materials assessment on the other. On the Brockley and Forest Hill terraces the side-extension rule is irrelevant. On the Catford and Hither Green semis it is the substantive loss.
Two-storey extensions almost always require full planning permission. Lewisham Council generally expects two-storey extensions to be subordinate to the original dwelling, set in from the side boundary, and designed to avoid unacceptable loss of light or privacy to neighbouring properties.
The council granted 1,221 certificates of lawfulness in three years (MHCLG planning statistics, retrieved 1 September 2026). In a borough this hard on deep extensions and this easy on everything else, the certificate is the instrument that keeps a project on the favourable side of that line - it records that the scheme sat within permitted development and never needed the consultation that refuses most of what reaches it.
Frequently asked questions
How much does a house extension cost in Lewisham?
Build costs for a house extension in Lewisham typically range from £2,500 to £3,800 per square metre, as Lewisham is an inner London borough. A typical single-storey rear extension of 15-20 sqm would cost between £37,500 and £76,000 for the build alone. On top of build costs.
Can I extend under Permitted Development in Lewisham?
Many single-storey rear extensions in Lewisham qualify as Permitted Development, allowing you to build without planning permission. You can extend up to 6 metres from the rear wall for terraced and semi-detached houses, or up to 8 metres for detached houses under Prior Approval. However, properties in Lewisham's 30 conservation areas have more restricted PD rights. We recommend applying for a Lawful Development Certificate (£274) to confirm PD eligibility.
How long does a house extension take in Lewisham?
A typical house extension project in Lewisham takes 5-8 months from initial instruction to completion. This breaks down as: 2-3 weeks for measured survey and drawing preparation, 8 weeks for planning determination by Lewisham Council (if required), 2-4 weeks for building regulations approval, and 8-16 weeks for construction depending on the size and complexity of the extension.
Do I need a Party Wall Agreement for an extension in Lewisham?
Terraced Lewisham property means two adjoining owners, semi-detached means one. Rear extension footings within three metres of a neighbour's foundation at a lower depth need a month's notice; building on the boundary line needs two. The Act is civil law between neighbours and runs entirely separately from whether Lewisham needs to see the drawings.
What experience do you have with extensions in Lewisham?
We prepare extension drawings for properties across all Lewisham postcodes (SE3, SE4, SE6, SE8, SE12, SE13, SE14, SE23, SE26). Our MCIAT chartered architectural technologists understand Lewisham Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
