Planning an extension is exciting, but it’s easy to get stuck on one key question: “Do I need planning permission?” The short answer is, it depends. At Draw Architecture, we work with homeowners across London who often underestimate how rules differ based on size, layout, and property type. A mistake here can delay your build or force costly changes. That’s why we’ve created this 2025 guide, to help you understand exactly when planning permission is needed, what rules apply to your type of extension, and how to avoid common issues from the start.
Planning permission is official consent from your local council that allows you to extend or alter your home. In most cases, it's needed if your project significantly changes the structure, size, or external appearance of your property. However, under permitted development rights, many smaller extensions can be done without submitting a formal planning application.
In 2025, planning policy remains heavily shaped by local borough guidelines, especially across London. So while national rules offer a foundation, it's your local authority's interpretation that matters most. Councils like Camden, Wandsworth, and Lambeth each have specific height limits, conservation rules, and restrictions you need to meet. This makes it essential to work with professionals who understand your borough’s planning expectations.
Whether you’re building out to the rear, to the side, or upwards, the type and size of your extension will determine if permission is required, and we’ll break down each scenario next.
Many homeowners are surprised to learn they don’t always need planning permission. That’s because of something called permitted development rights, a national set of rules that allow you to extend your home without a full planning application, as long as your design stays within strict size and location limits.
For example, you can typically add a rear single-storey extension up to 3 metres (for terraced and semi-detached homes) or 4 metres (for detached homes), measured from the original rear wall. Your extension must not cover more than half the land around your original house, and it must not be higher than 4 metres at the ridge or 3 metres at the eaves.
But these rights don’t apply everywhere. If your property is in a conservation area, national park, or affected by an Article 4 Direction, permitted development rights may be removed. Also, flats and maisonettes are not covered by these rights at all.
At Draw Architecture, we start every project with a detailed planning check to confirm if your extension is covered under permitted development or if we need to prepare a full planning application.
You’ll need planning permission if your extension goes beyond permitted development limits, affects the building’s appearance in a major way, or is located in a restricted area. Most two-storey extensions and many side extensions in London will require formal approval. Below are some common situations where planning permission is usually required:
If your side extension is more than half the width of your house or if you live on a corner plot (where your extension is visible from the street), you’ll likely need permission. Councils may be stricter with side extensions because they affect the streetscape.
Any extension that adds a new storey to your home or changes the roof height needs planning permission. Even if your neighbour has a similar extension, that doesn’t guarantee yours will be approved, every site is judged individually.
Permitted development rights do not apply to flats or maisonettes. Any extension, including balconies or external stairs, requires full planning consent.
If your home is listed or in a conservation area, or if your local council has applied an Article 4 Direction, even minor changes will need approval. We’ve worked on many such properties across London and can guide you through the added paperwork and design limitations involved.
Different extension types come with different planning rules. Some fit within permitted development limits, while others don’t. Here's how the rules break down:
This is the most common type and often covered under permitted development, but size matters. Extensions must not exceed 3 metres (terraced/semi) or 4 metres (detached), and the total height cannot go over 4 metres. Anything larger typically requires prior approval or full planning permission.
Side return extensions, often used to expand kitchens, can be tricky. If your extension is close to a boundary or wraps around, it may go beyond permitted development. If it's visible from the street, permission is more likely to be required.
Converting a garage into living space may not need planning permission if the building remains largely unchanged externally. But if you’re building over or replacing the garage structure, permission is usually needed, especially in conservation areas.
These combine side and rear extensions and are not typically covered under permitted development. Planning permission is nearly always required because the total volume exceeds what’s allowed without approval.
Even if you don’t need planning permission, you almost always need to follow building regulations. These are national standards that make sure buildings are safe, energy-efficient, and accessible. They cover things like structural stability, fire safety, ventilation, damp proofing, insulation, and drainage.
For any extension that creates new habitable space, you’ll need approval under building regulations. That includes adding a kitchen, bedroom, or any room people spend time in. Your project will likely require structural calculations (usually by an engineer), proper drawings, and on-site inspections from a building control officer during construction.
Since 2022, there’s also Part O to consider. It focuses on reducing overheating in homes and applies mostly to new builds, but in dense urban areas like London, some extensions must show they won’t trap too much heat, especially if you’re adding large windows or south-facing glass.
At Draw Architecture, we include building regulations packages for all relevant projects. We prepare the documents, liaise with consultants, and make sure everything is ready for construction sign-off.
Prior approval is a fast-track planning route for larger extensions that still fall within permitted development, but which require extra checks. If you’re planning a rear extension between 3 and 6 metres for terraced/semi-detached homes (or up to 8 metres for detached houses), prior approval is required.
It’s not a full planning application, but the council will ask for plans and notify your neighbours. They have 42 days to raise objections. If no valid concerns are raised or if the council doesn’t respond in time, you can start work.
While it’s simpler than full planning, it’s still a legal process. If you build without getting prior approval when it’s required, the council can take enforcement action.
At Draw Architecture, we regularly help clients navigate this route, preparing clean, accurate drawings and submitting them to councils on your behalf. We also advise on design changes to reduce the risk of neighbour objections.
If your extension work involves digging foundations near a shared wall or building against a boundary, the Party Wall Act may apply. This is a separate legal requirement from planning permission or building control. It’s meant to protect both you and your neighbour by setting out agreed terms for the work.
You’ll need to serve a formal Party Wall Notice to your neighbours. If they agree, work can go ahead. If they dissent, you’ll both appoint surveyors to draw up a Party Wall Agreement. This process can take weeks, so it’s wise to allow extra time in your build schedule.
Common extension triggers for this Act include:
We often coordinate with Party Wall surveyors for our clients and flag early on if this applies to your project.
The basic planning application fee for a householder extension in England is currently £258 (as of 2025). But that’s just the fee paid to the local council. There are usually other costs to consider:
Total costs vary depending on the property and how much documentation your local council expects. In some cases, applying for prior approval rather than full planning can save money, but it still involves professional fees.
At Draw Architecture, we provide upfront estimates for all planning-related costs. We manage the full process, from concept designs through to submission, and keep you informed every step of the way.
It can be tempting to start building before permission is granted, especially if neighbours seem relaxed or similar work has been done nearby. But building without the correct permissions is a big risk. Councils can issue an enforcement notice, which may force you to:
Retrospective applications are not guaranteed to succeed, especially if your extension breaches local policy. Even if your build is structurally sound and well-designed, it could still be rejected.
If enforcement action is taken and you don’t comply, you could face legal action or fines. It’s always better to check before you build.
If you're unsure whether you need permission, Draw Architecture can carry out a planning review of your project and advise the best route forward, avoiding problems before they arise.
At Draw Architecture, we understand how confusing the extension process can feel. That’s why we offer Architecture Services, precision, and full guidance from start to finish. With deep knowledge of London boroughs and planning policies, we help homeowners avoid delays and design issues before they happen.
Here’s what sets us apart:
When you work with Draw Architecture, you’re not just getting drawings. You’re getting a design partner who helps turn your ideas into a built reality, without surprises.
Before you commit to starting your extension project, run through this quick checklist:
✔️ Does your project fall under permitted development?
✔️ Have you checked local borough rules or conservation restrictions?
✔️ Will your extension need prior approval for size?
✔️ Have you considered building regulations and arranged for drawings?
✔️ Are you working close to a boundary, needing Party Wall notices?
✔️ Have you budgeted for planning fees and potential surveyor costs?
✔️ Do you need a planning consultant or architect’s help with the process?
If you answered “no” or “not sure” to any of the above, it’s a good idea to speak to a professional. Draw Architecture can help you confirm where you stand, and plan the next steps confidently.
Most householder planning applications take 8 weeks for a decision after submission. If the council requires extra reports or consultations, it may take longer. Prior approval decisions usually come within 42 days.
Yes, if your extension meets permitted development rules, and your house is not in a restricted area. This typically applies to small rear extensions and some side extensions. Always check first, rules are detailed.
Neighbour comments are reviewed during the planning or prior approval process. While councils consider objections, they don’t automatically reject a project. Design changes can sometimes resolve issues early.
Yes, but most people hire an architect to prepare professional drawings and handle the submission. Errors in plans or missing documents can delay the process or result in rejection.
Yes. Even if planning permission isn’t required, building control approval almost always is. This ensures the work is structurally safe and legally compliant.
Architectural advice, including planning permission and fee estimates, can vary depending on the local authority, property type, and specific requirements of each project. While this article is intended as a helpful guide, we strongly recommend confirming your situation with a qualified architect or planning consultant. All information is correct at the time of writing (2025) but is subject to change under UK planning law.
