TL;DR — Key Facts
- Build cost per sqm: £2,200 – £3,200 (outer London borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 28 designated areas in Merton
- Permitted Development: Generally available outside conservation areas
Extension costs in Merton
Costs here are build costs. For the drawing package itself — scope, fee and timeline — see house extensions in Merton.
House extension build costs in Merton typically range from £2,200 to £3,200 per square metre, reflecting Merton's position as an outer London borough. These figures cover the construction cost only and do not include professional fees, planning fees, or VAT.
Merton's range is set by which half of the borough the property is in. Wimbledon's Victorian villas carry the heaviest conservation coverage here, so matched materials and considered detailing are the cost drivers. Morden and Raynes Park semis are conventional structural jobs. The Edwardian terraces towards Tooting sit between the two, with narrow plots that make the structural opening proportionally larger.
Typical extension costs in Merton (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 Merton
Merton issued 156 certificates of lawful development in three years, the third-lowest count in London (MHCLG planning statistics, retrieved 1 September 2026), which is surprising for a borough with no Article 4 Direction and a permissive record. It suggests homeowners here apply where they could certify. The stock supports both routes: Wimbledon's villas have space, Morden and Raynes Park's semis have side access, and Tooting's terraces have neither.
Conservation areas
Merton has 28 designated conservation areas, including Wimbledon Village, John Innes, Mitcham Cricket Green, Wimbledon Common. 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 Merton is predominantly victorian villas in wimbledon; edwardian terraces in tooting and colliers wood; 1930s semi-detached in morden and raynes park; post-war flats in mitcham. 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
Wimbledon's Victorian villas frequently allow plant to the rear and are the least constrained sites in the borough, though they carry the heaviest conservation coverage. Morden and Raynes Park semis have the standard side passage. The Edwardian terraces towards Tooting and Colliers Wood require everything to pass through the house, and Mitcham's post-war flats are not extension candidates at all.
Our drawing fees for Merton extensions
At Architectural Drawings London. No hourly billing, no surprises. Our fees cover everything needed for a valid planning submission to Merton Council and for building regulations approval.
Drawing fees for extensions in Merton
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 Merton Council on your behalf.
We cover all Merton postcodes: SW16, SW17, SW19, SW20, CR4.
Planning vs Permitted Development in Merton
Many house extensions in Merton 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).
Merton's 28 conservation areas are weighted heavily towards Wimbledon. There, on Victorian villas with generous plots, losing the side-extension right is a genuine constraint. Towards Tooting and Colliers Wood the stock is terraced and the materials rule governs instead. Morden and Raynes Park sit largely outside designation altogether.
Two-storey extensions need an application. Merton divides between Wimbledon, where the villas have generous plots but the heaviest conservation coverage in the borough, and Morden and Raynes Park, where interwar semis have workable side space and far less designation. The council also handles more listed-building work than most boroughs of its size, so status is worth confirming before design.
At 156 certificates over three years this borough uses the route far less than its neighbours, and that is worth reconsidering rather than copying. A certificate costs a fraction of an application, is decided by officers on the facts rather than on the merits, and produces the document a purchaser's solicitor will ask for. We prepare the drawings for one from the same survey.
Frequently asked questions
How much does a house extension cost in Merton?
Build costs for a house extension in Merton typically range from £2,200 to £3,200 per square metre, as Merton is an outer London borough. A typical single-storey rear extension of 15-20 sqm would cost between £33,000 and £64,000 for the build alone. On top of build costs.
Can I extend under Permitted Development in Merton?
Many single-storey rear extensions in Merton 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 Merton's 28 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 Merton?
A typical house extension project in Merton 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 Merton 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 Merton?
One neighbour on the semi-detached stock, two on the Tooting terraces. Excavation within three metres of an adjoining foundation is the usual trigger on a rear extension. Where a Merton property is listed - the council issued eight listed-building works certificates in three years, third in London behind Harrow and Westminster (MHCLG planning statistics, retrieved 1 September 2026) - listed building consent is an additional and entirely separate requirement.
What experience do you have with extensions in Merton?
We prepare extension drawings for properties across all Merton postcodes (SW16, SW17, SW19, SW20, CR4). Our MCIAT chartered architectural technologists understand Merton Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
