Brent Extension Guide 2026: Planning Permission, Costs & Local Rules

Brent: The Borough and Its Housing Stock

Brent sits in north-west London, covering a wide arc of housing types built across different eras. Victorian terraces dominate Kilburn and Kensal Rise in the south; Edwardian semis and the Arts and Crafts-influenced streets of Brondesbury fill the middle belt; 1930s suburban semis and detached houses characterise Wembley, Kenton, and Preston to the north.

This variety means extension strategies differ markedly by neighbourhood. A Kilburn Victorian terrace calls for a different approach — and faces different planning constraints — than a Wembley 1930s semi. Understanding where your property sits within Brent's planning geography is the essential first step.

Brent is not one of London's most restrictive boroughs, but it is not permissive either. The borough has designated fourteen conservation areas, all carrying Article 4 directions, and in January 2025 it updated its supplementary planning guidance for the first time since 2018. That new SPD now governs how applications for extensions are assessed.

The New Extensions SPD: What Changed in January 2025

Brent adopted its updated Residential Extensions and Alterations SPD on 16 January 2025, with the new guidance taking effect from 25 January 2025. It replaced the 2018 version and is now the primary design reference alongside the Local Plan for any extension application in the borough.

The council's stated aim was to allow residents to make better use of existing housing stock while ensuring extensions respect the character of Brent's neighbourhoods. The new SPD clarifies design expectations and strengthens requirements around trees, biodiversity, and sustainability.

Key points from the 2025 SPD

• Two-storey rear depth limits: Maximum 3m depth from the original rear wall for attached homes, 4m for detached homes — the eaves and ridgeline must not exceed those of the original building
• Tree retention: Existing trees must be retained where possible; where removal is unavoidable, replacement planting on-site is expected
• Biodiversity: Applicants are encouraged to increase biodiversity through planting, water features, and retained wildlife habitats — particularly relevant for larger garden extensions
• Design coherence: Extensions should use materials, roof forms, and window proportions that complement the existing building and its street context
• Flooding guidance: Sustainable drainage considerations are addressed, particularly for basement-level or ground-floor extensions in areas with surface water flood risk

The SPD applies across the whole borough and is used by Brent officers when assessing all householder applications. Even if your extension falls within permitted development limits, the SPD design principles are good practice to follow — and are a requirement when planning permission is needed.

Conservation Areas in Brent

Brent has fourteen designated conservation areas, and every residential conservation area in the borough is covered by an Article 4 direction. This means that extensions which would normally be permitted development require a full planning application within these areas. The council confirmed new Article 4 directions in 2024 to replace the existing directions, including updated boundaries following conservation area extensions decided in May 2024.

Mapesbury Estate

Mapesbury is the most significant conservation area for homeowners considering extensions. It is one of the largest conservation areas in Brent and comprises a largely unaltered townhouse development built between 1895 and 1905, centred on the streets around Mapesbury Road in Brondesbury Park. The character is distinctly Edwardian, with red brick, bay windows, terracotta detailing, and Arts and Crafts-influenced roof forms.

The Mapesbury Conservation Area Design Guide provides detailed guidance on materials and design approach. The boundary was extended along Chichele Road and surrounding streets in May 2024, and new Article 4 directions cover the enlarged area. Pre-application advice is strongly recommended before preparing designs for any extension here.

Other Notable Conservation Areas

• Barn Hill: A distinctive inter-war suburb built around a nature reserve in Wembley Park. Designated in 1990, it has its own character appraisal. Article 4 direction applies. Extensions must respect the suburban Arts and Crafts character of the 1920s–30s houses.
• Brondesbury: Edwardian and late Victorian streets around Brondesbury Park station. Boundary extended along Cavendish Road and Chatsworth Road in May 2024. Article 4 direction applies to the enlarged area.
• Queen's Park: Victorian terraces around the park of the same name. Boundary extended along Salusbury Road and surrounding streets in May 2024. Article 4 direction applies.
• Kensal Rise: A newly designated conservation area created in May 2024, covering streets of Victorian and Edwardian housing north of the railway. Article 4 direction applies.
• Willesden Green and Harlesden: Further residential conservation areas in the central and eastern parts of the borough. Both have Article 4 directions.

Use our free tool to check whether your property is within a Brent conservation area or subject to an Article 4 direction before engaging an architect.

Permitted Development in Brent

Outside conservation areas and Article 4 zones, standard national permitted development rights apply. This covers much of Wembley, Kenton, and other areas of the borough not designated as conservation areas. The key rules for houses are:

For larger single-storey rear extensions (over 3m for terraced/semi-detached, over 4m for detached), the prior approval process applies nationally. Brent will notify neighbours and assess impact on amenity. This is faster and cheaper than full planning permission but still requires a formal application.

The Planning Process in Brent

Brent aims to determine standard householder planning applications within 8 weeks of validation. More complex applications — those involving heritage assets, larger footprints, or significant neighbour impact — can take longer in practice, particularly where additional information is requested mid-process.

Pre-application advice

Brent offers a pre-application advice service for householder proposals. This is discretionary but strongly advisable for any property in a conservation area, any extension involving a party wall with a listed structure, or any project that is pushing at the edges of what the new SPD describes. Officers will give a written response indicating the principle of development and likely issues, giving you the chance to refine designs before a formal submission.

For Mapesbury Estate in particular, a pre-application meeting is effectively essential. The area attracts high scrutiny from the conservation team, and an early steer on materials and design approach can prevent costly revisions later.

What to submit

• Scaled drawings: existing and proposed floor plans, elevations, and a location plan
• Design and access statement for conservation area applications, describing how the proposal responds to the character appraisal
• Materials schedule specifying brick type, roof tile, window material and glazing profile
• Tree survey if any trees on the site or adjacent land are affected by the works
• Householder application fee: £258 (from December 2024 uplift)

Design Requirements: Mapesbury Estate and Conservation Areas

Brent's conservation area design guides are more specific than the broad SPD guidance. For Mapesbury Estate, the character is defined by its consistent Edwardian townhouse form: two or three storeys, red brick with terracotta string courses, steeply pitched roofs, prominent bay windows, decorative timber gable detailing, and white-painted sash windows. The streetscape has survived largely intact, which is why the area was designated in the first place.

What this means for extensions

• Rear extensions: Typically more straightforward than side or front alterations. A single-storey rear extension in a complementary facing brick is generally acceptable if it does not dominate the garden or affect a shared amenity space.
• Materials: Red brick matching the original as closely as possible. Render, UPVC windows, and aluminium-framed glazing visible from the street are generally resisted.
• Rooflines: Extensions should not compromise the pitched roof character. Flat-roof additions at the rear are sometimes acceptable but should not be visible from public vantage points.
• Side returns: Many Mapesbury and Brondesbury properties have usable side return space. These extensions are feasible but require care with the glazing approach — a fully glazed lantern roof is often preferred over a solid extension at this scale.
• Loft conversions: Dormers to the rear are usually approvable. Front dormers in conservation areas are much more difficult — the council is protective of original front roofscapes visible from the street.

See our full guide to conservation area extensions in London for the design principles that apply borough-wide.

Extension Costs in Brent — 2026

All costs below include VAT at 20% and reflect 2026 north-west London contractor rates. Brent sits at the lower end of inner London pricing, with costs broadly in line with boroughs such as Ealing and Barnet rather than the premium rates of Camden or Kensington.

Professional fees and other costs

• Architect fees: £4,000–£12,000 (10–15% of build cost for a full service)
• Structural engineer: £1,200–£3,000
• Planning application fee: £258 (householder) from December 2024
• Party wall surveyor: £1,500–£3,000 per neighbour (required when works adjoin a shared boundary)
• Building Control: £900–£2,200 depending on project scope

For a full breakdown of London extension costs by size and type, see our London extension costs guide for 2026.

Common Extension Types in Brent by Area

Victorian terraces — Kilburn and south Brent

The dense Victorian terraces of Kilburn and Kensal Rise are typically suitable for single-storey rear extensions and side return extensions. The side return — the narrow strip of land alongside the kitchen that runs to the back of the plot — is a particularly common target. Infilling this space creates an open-plan kitchen-diner without dramatically extending into the garden. See our side return extension guide for detailed costs and design options. Many of these streets now fall within Queen's Park or Kensal Rise conservation areas, so planning permission is required.

Edwardian semis — Brondesbury and Mapesbury

The larger Edwardian semis of Brondesbury and the Mapesbury Estate have more garden depth and often wider plots, making full rear extensions more feasible. These properties also benefit from generous loft space — a hip-to-gable loft conversion combined with a rear dormer is a popular choice, adding a bedroom and bathroom without touching the garden. Planning is always required in these conservation areas. A Victorian and Edwardian house extension approach applies well here: sympathetic materials, subservient form, and considered fenestration.

1930s semis — Wembley, Kenton, and Preston

The 1930s semis that dominate the northern parts of the borough are among the most extension-friendly housing types in London. Many have side garage structures that can be converted or replaced with a two-storey side extension. The characteristic hip roof can be altered to a gable in a hip-to-gable loft conversion, adding significant headroom. Most of these streets are outside conservation areas, so permitted development rights often apply — though the new SPD guidance is relevant if planning is needed. See our rear extension guide for the full range of options for this property type.

Planning Tips for Brent Extensions

• Mapesbury Estate: always take pre-app advice. The conservation team assesses applications against the character appraisal and design guide closely. An early steer on materials and massing prevents refusals and expensive redesigns.
• The new SPD is applied consistently. Since January 2025 all applications are assessed against the updated document. Designs that complied with the 2018 SPD may need to be revisited if they have not yet been submitted.
• Check the conservation area boundary, not just the postcode. The May 2024 extensions to Mapesbury, Brondesbury, Queen's Park, and Willesden Green, plus the new Kensal Rise designation, mean streets that were outside these areas a year ago may now be inside them. Verify your property's status with the council or using our tool.
• Tree surveys add time — commission early. The SPD's strengthened emphasis on tree retention means that any project affecting roots or canopies will need an arboricultural report. These take two to four weeks to commission and are required at validation.
• Party wall notices: serve early. Many Brent properties share boundaries on multiple sides. Serving party wall notices two months before construction starts is advisable; neighbours have two months to respond or appoint a surveyor.
• Permitted development outside conservation areas: still check with the council. Even where PD rights technically apply, any ambiguity about boundary, height, or proximity to a neighbouring property is worth clarifying with a Lawful Development Certificate before construction starts.