Planning a Residential Extension in London? What You Need to Know Before Hiring an Architect

(Private Residential | Visuals produced by E2 for Planning and Listed Building Consent Applications. The proposals followed principles outlined in our Architect Home Visit. They includes alterations and single storey extension to a Grade II listed building in the Charlton Village Conservation Area. Full Householder Planning Permission and Listed Building Consent have been granted. Learn more about this project)

The rise of home extensions 

Thinking about extending your London home? Before you dive in, it’s essential to understand planning rules, the risks of building without permission, and how the right architect can save you time, stress, and money.

There has been a noticeable increase in home extensions across London as homeowners look to expand their living space and transform their properties to suit their lifestyles.

This trend is driven by a combination of factors including limited space, rising house prices, the cost of moving (and stamp duty) and the desire for more flexible living spaces.

Since COVID, the demand for home offices and larger living areas has significantly increased, with home extensions offering a cost-effective way to gain space without relocating and adding value in the process.

Many homeowners assume that extensions fall under Permitted Development, where home improvements and extensions can be carried out without the need to apply for planning permission – but this isn’t always true.

If you’re planning a residential extension in London, then this blog is for you. We will break down when planning permission is needed for a home extension and how to extend legally and efficiently. Vital information to have at hand before deciding on an Architect.

(Private Residential | Visuals by E2 for the proposed alterations and single storey extension of a 17th Century Grade II listed lobby entry house in the South Mimms Conservation Area. Planning permission advice was sought prior to Full Householder Planning Permission and Listed Building Consent Applciations. Learn more about this project)

What are the common types of residential extensions?

The most common types of residential extensions include:

• Single-storey rear extensions – often to create an open-plan living and kitchen space or to add a dining space.
• Loft extensions – to add a bedroom or shower room, or more recently the addition of a home office.
• Side-return extensions – especially popular in older homes.
• Wrap-around extensions – combine rear and side extensions for a significant increase in living space.
• Basement extensions – opens up a significant amount of unused space. However basement extensions are almost always unviable unless in a very high value area (for example in the boroughs of Chelsea, Kensington).

Which type of extension does and doesn’t fall under Permitted Development.

In London, single-storey rear and side extensions, loft conversions, and certain outbuildings generally fall under Permitted Development if they meet specific size and height restrictions.

However, two-story extensions, wraparound extensions (combining side and rear), and extensions exceeding Permitted Development limits (e.g. beyond 3m for semi-detached, 4m for detached) generally require planning permission.

Also, flats, listed buildings, and properties in conservation areas often have more stringent rules.

Permitted Development (Generally Allowed):

• Single-Storey Rear Extensions – Up to 3m deep for semi-detached/terraced, and 4m for detached, with height restrictions (e.g., 4m total, eaves no higher than 3m if near a boundary).
• Side Extensions – Up to half the width of the original house, and must not extend beyond the rear wall of the house.
• Loft Conversions – Up to 40 cubic meters for terraced houses, and 50 cubic meters for detached and semi-detached.
• Outbuildings – Certain sheds and garden rooms can be built without planning permission, as long as they are not used as residential accommodation.
• Porches – Not exceeding 3m in height, and no more than 2m close to any boundary next to a highway.

Not Permitted Development (Generally Requires Planning Permission):

• Two-Storey Side Extensions – These extensions typically require planning permission.
• Wraparound Extensions – Combining side and rear extensions often exceeds Permitted Development limits and needs permission.
• Larger Rear Extensions – Extensions beyond the 3m/4m (semi-detached/detached) limits generally require a neighbour consultation scheme or full planning permission.
• Flats and Maisonettes – These generally do not benefit from Permitted Development rights.
• Listed Buildings – Any alterations or extensions to listed buildings require  listed building consent, regardless of whether planning permission is also needed.
• Properties in Conservation Areas – These may have stricter rules and require planning permission for extensions that would normally be permitted. Often there are ‘Article 4 directions’, where the local planning authority can remove specific Permitted Development rights in their conservation area.
• Extensions exceeding height restrictions – Any extension exceeding the height restrictions, such as those above the existing roofline or exceeding 4m in total height. Height restrictions are also related to the proximity to the boundary, within 2m of the boundary you can’t exceed 3m height. Given most London properties are 5m wide, most of the extensions have to be under 3m in height.

For further guidance on Permitted Development and building regulations for projects around the home, visit the Interactive House on the Planning Portal website.

(Private Residential | Royal Hill, Initial Architect Home Visit sketch option. To meet the client brief, the proposals were for alterations to thier Grade II listed property with a single storey rear extension design and a new outbuilidng wthin the garden. Full Householder Planning Permission and Listed Building Consent have been granted with all conditions discharged. Learn more about this project)

Important considerations, alongside Permitted Development:

Even if your extension does fall under Permitted Development, we recommend you apply for a certificate of lawful development from your local council. That way the council can highlight if there is any issue and if all is ok, you have the certificate, confidence and paper trail.

For some larger rear extensions, a “prior approval” process might be needed, involving a neighbour consultation scheme.

It’s also essential to consider UK Building Regulations – regardless of whether planning permission is needed, building regulations approval might still be required for structural aspects of the extension.

An important section of the Building Regulations that is worth looking at is the Approved Document L which provides guidance on the conservation of fuel and power. This includes specific guidelines around how much glazing you can have in an extension. Although everyone is keen on glass, there are good reasons for restricting the amount of glass, associated with energy efficiency and comfort.

It is crucial to check with the local planning authority for specific rules and any potential restrictions in the area.

Building without Planning Permission: The risks

There are a number of consequences that you need to be aware of if you build without receiving the necessary permission. These include:

Enforcement Notices:

Local councils can force you to undo the work – they can serve you with an enforcement notice to stop work and to make an application for planning consent. If that consent is not granted, then they can serve you with an enforcement notice to demolish what you’ve built without consent.

Problems with Selling Your Property:

Lack of certification can delay or derail the sale of your property. If you’re trying to sell a property without the correct planning permission, this will likely be identified in the conveyancing.

Retrospective Planning Applications:

These can be refused, leaving you in legal limbo and possibly having to demolish what you’ve already built.

Resale Problems:

Buyers and solicitors will flag unpermitted work, causing delays or failed sales.

Change of Use and Other Considerations

Converting garages, outbuildings, or commercial units into homes often requires Change of Use Planning Permission.

For example, turning a garage into a habitable room or splitting a single-family home into multiple flats usually falls outside Permitted Development. This is because you will likely need to make other changes, for example fill in the garage door with a proper wall or need an additional door. It’s important to check whether those additional alterations fall under Permitted Development or not, particularly if in a conservation area or listed building.

If you’re considering this type of extension, it’s important to seek planning permission advice early. Getting this wrong can result in delays, costly appeals, or even enforcement action later.

(Private Residential | Royal Hill, planning application visual of the proposed outbuilding in the rear garden to replace and improve the existing planning permission. The outbuilding’s architectural language was in conjunction with the house extension design to connect with each other nestled in the setting of the Greenwich Conservation Area. Learn more about this project)

Do You Need an Architect for an Extension?

A common question we get asked is: “Do I need an architect for an extension?”

The short answer is yes.

If it’s a relatively simple Permitted Development type extension, with limited design aspirations, you may be ok to go with a local design and build company that specialise in the type of extension you want. This can offer a quick and economical route.

However, if the project involves any complexity (trying to push the boundaries, listed building or conservation area for example), it’s better to at least do the initial design stages up to planning stage with an Architect.

If you have quality and aspirations, then continuing with the Architect to design and build stage is advised.

A qualified architect not only produces accurate drawings but also ensures your project complies with planning rules and Building Regulations. Choosing experienced residential architects in London means you’ll:

• Maximise space with creative house extension designs.
• Navigate local planning rules confidently.
• Reduce the risk of retrospective planning applications being refused.
• Improve design appeal and property value.
• Receive support with budgeting and project management.

Ultimately, hiring an architect is an investment in avoiding mistakes, saving time, and creating a better home.

(Private Residential | Royal Hill, concept design stage visuals. Interior view of proposed extension architecture. Following the Architect’s Home Visit sketch design, E2 developed the house extension design with the clients every step of the way to affirm the concept’s general arrangement inside and out. Additionally, the clients opted in for our in-house visualisation service to understand the impact of the proposals  Learn more about this project)

Design Ideas and Case Studies

When it comes to extension architecture, the possibilities are huge.

At E2, we’ve designed a range of successful house extensions across London, including the boroughs of Blackheath, Greenwich, Hackney and Camden. Each project relies on our proven ability to manage both the design and planning approval process.

Royal Hill, Greenwich – Extension to a Listed Building:

In Greenwich, E2 restored and extended a Grade II listed house by adding a single-storey rear extension and a garden studio. Although within Permitted Development limits, we chose enduring timber cladding and crittall glazing to compliment the original render. The design also includes a green roof, reclaimed brick terraces, and careful repairs to the front façade and gate. By treating every element, from materials to elevation, with respect for the building’s heritage, we secured listed building consent and delivered a modern, light-filled home that also enhances the conservation area.

(Private Residential | Royal Hill, rear extension design and construction completed early 2025. Contemporary kitchen with natural earthy tones. Finishes include Living Concrete Microcement floor, Clay plaster and paint finish to the walls, Birch Ply Ceiling with Oak exposed joists. PPC double glazed door and windows by Archglazing Ltd, DeVol Kitchen joinery supplied by clients. Learn more about this project)

Medusa Road, Catford – Rear extension to create a dining room

In Catford, we helped a client get a highly successful rear extension to a Victorian terrace with a carefully considered full-width dining/kitchen addition. Though the design used zinc cladding instead of traditional brick, which meant it wasn’t automatically covered under Permitted Development – we leveraged existing zinc elements on the roof and worked through a robust feasibility process including our architect home visit to ensure the design suited the house and site.

The result: a striking, contemporary extension that enhances the lower-ground floor’s flow and garden views, while still respecting the character of the original building.

(Private Residential | Proposed single storey extension with contemporary design. Early feasibility sketches with the client during their architect home visit explored the benefits and virtues of extending to the full width of their Victorian terraced house. Learn more about this project)

Primrose Hill – Roof extension & incorporated balcony

In Primrose Hill, we took on a flat- adding a bold roof extension far beyond what Permitted Development would allow. The twisted bonus? A balcony with anthracite zinc cladding and glazed balustrade. We flipped the layout so the new roof-level became the kitchen & dining, and converted the old kitchen into a bedroom – creating an “upside-down” flat that works beautifully. Materials like reclaimed timber floors and exposed joists help the addition feel rooted in the original architecture, even while it stands out.

(Private Residential | Roof extension with an outdoor space  – The property was reconfigured to provide two generous bedrooms with large ensuite bathrooms. The materials used on this project centred around creating a space that gave the impression that it had been a part of the building all along... Learn more about this project)

How E2 Architecture Can Help

At E2, we specialise in helping London homeowners unlock their property’s full potential. As conservation and residential architects London, we have extensive experience in:

• Designing extensions that respect heritage and conservation requirements and dealing with listed building.
• Managing planning applications and approvals, including complex cases.
• Supporting you through the process, from initial architect home visits to final build.
• Providing clear planning permission advice tailored to your property and borough.

We’ve often helped clients succeed in getting listed building consent where others have failed and had planning permission refused.