How to Get Planning Permission for a Rear Extension in London

A loft conversion is one of the most cost-effective ways to add a bedroom, bathroom, or home office to a London property without extending the footprint of the building. But the right type of loft conversion depends on your roof structure, the amount of headroom you have, and whether you need planning permission.

The most common type is the dormer loft conversion. A dormer is a structural addition that projects vertically from the slope of the roof, creating a box-like extension with a flat or pitched roof. Dormers dramatically increase usable floor space and headroom, and are suitable for most terraced, semi-detached, and detached houses. Rear dormers often fall within Permitted Development rights, meaning no planning application is required — though conservation area properties are excluded.

A hip-to-gable conversion is suitable for semi-detached and detached houses with a hipped roof — where the roof slopes on all four sides. The conversion involves extending the sloping hip end of the roof to create a vertical gable wall, significantly increasing the internal volume of the loft. Hip-to-gable conversions are often combined with a rear dormer to maximise space.

The mansard conversion is the most substantial type and involves replacing the entire rear roof slope with a near-vertical wall (typically at 72 degrees) and a flat roof. Mansard conversions are common in London terraced houses and can create a full-width, full-height room across the rear of the property. Because they alter the roofline significantly, mansard conversions almost always require planning permission — but they offer the greatest increase in floor area.

Whichever type you choose, a loft conversion must comply with Building Regulations covering structural calculations, fire escape routes, insulation, and staircase design. At Megaplan, we design and manage loft conversions from initial feasibility through to building control sign-off, ensuring your project is delivered on time and to the highest standard.

Many homeowners are surprised to learn that planning permission and Building Regulations are entirely separate approvals. You can have planning permission but still need Building Regulations approval before work begins — and in some cases (such as Permitted Development extensions), you need Building Regulations even though no planning application was required.

Building Regulations set minimum standards for the design and construction of buildings in England. For extensions, the key areas covered include: structural integrity (foundations, walls, floors, and roof); fire safety (escape routes, fire doors, smoke alarms); thermal performance (insulation to meet Part L energy efficiency requirements); ventilation (Part F); drainage and waterproofing; and electrical and plumbing installations.

There are two routes to Building Regulations approval. The Full Plans route involves submitting detailed drawings and specifications to the building control body before work starts. The building control officer reviews the plans and issues an approval notice. This is the route we recommend for most extension projects, as it gives you certainty before work begins and protects you if any disputes arise during construction.

The Building Notice route allows work to start without submitting full plans, with the building control officer inspecting the work at key stages. This route is faster but carries more risk — if the inspector requires changes, they must be made at the contractor's expense.

Since the Building Safety Act 2022, the regulatory framework for higher-risk buildings (those over 18 metres or 7 storeys) has been significantly strengthened. For most residential extensions in London, the changes most relevant to homeowners are the increased emphasis on the Golden Thread of information — maintaining accurate, up-to-date records of the building throughout its life.

At Megaplan, we prepare full plans Building Regulations submissions for all extension and loft conversion projects. We liaise directly with the building control body, respond to any technical queries, and attend site inspections to ensure your project achieves a completion certificate — the document you will need when you come to sell your property.

London has over 1,000 conservation areas — designated zones where the character and appearance of the area is considered worth preserving or enhancing. If your property is in a conservation area, the rules around extensions and alterations are stricter than for properties outside these zones.

The most significant difference is that Permitted Development rights are restricted in conservation areas. Works that would normally be allowed without planning permission — such as rear extensions, roof alterations, and cladding — require a planning application in a conservation area. This means that even a modest single-storey rear extension will need formal approval from your London borough council.

Conservation area planning applications are assessed against the council's Conservation Area Appraisal and Management Plan, which sets out the special character of the area and the design principles that new development should follow. Key considerations include the choice of materials (brick, stone, render, and roof tiles should match or complement the existing building and the character of the area), the scale and massing of the extension, and the impact on the streetscene and on neighbouring properties.

Pre-application advice is particularly valuable for conservation area projects. A pre-application meeting with the council's conservation officer can help you understand what is likely to be acceptable before you invest in detailed design work. At Megaplan, we have extensive experience of working in London conservation areas and can advise on the design approach most likely to secure approval.

If your property is a listed building as well as being in a conservation area, you will also need Listed Building Consent for any works that affect the character of the building — including internal alterations. Listed Building Consent is a separate application from planning permission and requires a heritage impact assessment prepared by a suitably qualified professional.

We have successfully secured planning permission for extensions and alterations in conservation areas across London, including projects in Lambeth, Southwark, Islington, and Hackney. Our approach combines a thorough understanding of local planning policy with high-quality architectural design — giving your application the best possible chance of a first-time approval.