Extending your home can add valuable space and improve your property’s usability, but many UK homeowners are unsure about planning permission rules. Getting this wrong can lead to costly mistakes, delays, or even orders to alter or remove the extension. The rules on planning permission have clear guidelines, but they can still be confusing because they vary based on the type of extension, your property’s location, and any restrictions placed by your local council. At Formi Architecture, we help clients understand these requirements from the start so projects run smoothly and within legal limits. This guide explains when you do and don’t need planning permission in 2025, how permitted development works, the specific rules for different extension types, and the costs and processes involved. By the end, you’ll have a clear idea of the steps to take before starting work on your home.
Understanding Planning Permission for Extensions in the UK
Planning permission is official approval from your local council that allows you to carry out building work. It ensures that the proposed extension meets national and local rules on design, safety, and how it affects neighbouring properties. In the UK, not all extensions require planning permission, but failing to check could result in enforcement action. As of 2025, planning laws remain based on long-standing guidelines but have been updated in some areas, such as environmental performance and design limits for certain builds. Local councils can also apply extra restrictions for specific neighbourhoods.
Getting planning permission right from the start is vital. It avoids the risk of having to submit retrospective applications or make costly changes. It also ensures that your project fits within community standards and enhances rather than harms the local area. For many homeowners, working with experienced architecture consultants like Formi Architecture offers peace of mind, as we assess your project’s compliance before any drawings are finalised.
When You Don’t Need Planning Permission – Permitted Development Explained
Permitted development rights are rules that allow certain types of building work to be carried out without a full planning application. They apply to many houses in the UK but exclude flats, maisonettes, and some properties in restricted areas. Under permitted development, you can often build small extensions without formal permission, provided you follow specific limits on height, depth, and design.
For example, a single-storey rear extension can typically extend up to 3 metres from the original house for terraced and semi-detached homes, and up to 4 metres for detached houses, without requiring planning permission. If the extension meets the larger home extension limits of up to 6 metres (semi-detached/terraced) or 8 metres (detached), you will still need to follow the prior approval process. There are also height restrictions: the extension cannot be higher than the existing roof and side extensions must be less than half the width of the original house.
At Formi Architecture, we guide clients through these rules, checking whether their planned work meets permitted development limits. This step ensures you avoid unnecessary applications while staying within legal boundaries.
When You Do Need Planning Permission for an Extension
Even with permitted development rights, there are many cases where you must apply for planning permission. Understanding these scenarios helps avoid delays and legal issues.
Side Extensions and Corner Plot Homes
Side extensions often require planning permission, especially if they are more than one storey high or more than half the width of the original property. Homes on corner plots may face stricter rules because the extension is visible from multiple directions and can affect the street view.
Two-Storey Extensions and Roof Alterations
Two-storey rear extensions almost always require planning permission due to height, light, and privacy impacts. Roof alterations, including dormers or changes to roof shape, can also trigger permission needs.
Flats, Maisonettes, and Split-Level Homes
Permitted development rights do not apply to these property types, meaning all extensions require formal planning approval.
Homes in Conservation Areas and Listed Buildings
If your property is in a conservation area, national park, or is a listed building, you will likely face additional restrictions under Article 4 Directions. These remove or limit permitted development rights, meaning you will need permission for even minor changes.
Planning Permission Rules for Common Extension Types
Planning rules differ depending on the type of extension you’re building. Understanding the specific requirements for each will help you plan your project effectively and avoid costly delays.
Single-Storey Rear Extensions
In most areas, a single-storey rear extension can fall under permitted development if it stays within size limits. For terraced and semi-detached homes, this means a maximum of 3 metres in depth, and for detached homes, up to 4 metres. Larger extensions of up to 6 or 8 metres may be possible through the prior approval process, but exceeding these dimensions will require planning permission. Height restrictions also apply; the extension must not be taller than 4 metres at the ridge. At Formi Architecture, we assess your plans to confirm whether they meet these limits or require an application, ensuring no surprises later in the process.
Kitchen Extensions and Side Returns
Kitchen extensions, including side return designs, can improve space and light but often have more restrictions due to proximity to boundaries. If the extension increases the house’s width by more than half, or if it affects neighbours’ daylight or privacy, planning permission will be needed. We guide clients on how to adjust designs to avoid unnecessary applications.
Garage Extensions or Conversions
Converting or extending a garage may require planning permission, particularly if it changes the building’s external appearance, increases its footprint, or changes its use. For example, turning a detached garage into a habitable room might require both planning permission and building regulations approval.
Wraparound and L-Shape Extensions
These combine rear and side extensions into a single build and almost always require planning permission because they exceed permitted development size limits. Our team prepares designs that balance your space needs with planning requirements to improve the chance of approval.
Do You Need Building Regulations for an Extension?
Building regulations are separate from planning permission and apply to almost all extensions. They set the technical standards for structural safety, fire protection, insulation, drainage, ventilation, and accessibility. Even if your extension qualifies under permitted development and avoids planning permission, it will still need to comply with building regulations.
In 2025, there is increased emphasis on energy efficiency and sustainability, with stricter insulation and glazing standards. Part O of the building regulations now addresses overheating in new homes and extensions with large glazed areas. This means that if your extension has extensive windows or glass doors, you must show it won’t lead to excessive heat build-up during warmer months.
Formi Architecture manages the building regulations process from drawings to liaison with building control officers. This ensures your extension meets all necessary standards before work begins, reducing the risk of costly alterations later.
Prior Approval and the Neighbour Consultation Scheme
Prior approval is a process for certain larger extensions that exceed standard permitted development limits but may still be allowed without full planning permission. This applies to rear extensions up to 6 metres for terraced or semi-detached houses, and up to 8 metres for detached houses.
Under the neighbour consultation scheme, your local council will inform neighbours about your plans. They have the opportunity to raise concerns about the extension’s size, light impact, or privacy issues. The council then decides whether the extension can proceed. The process usually takes around 42 days.
At Formi Architecture, we prepare clear plans and supporting information to address common objections and improve the likelihood of approval. We also advise clients on how to adjust designs to reduce potential disputes.
The Party Wall Act and Your Extension
The Party Wall Act protects both you and your neighbours when building work affects a shared wall, boundary wall, or structure close to a neighbour’s property. If your extension involves work on a party wall or within certain distances of a neighbouring property, you are required by law to give written notice in advance.
This applies to foundations near a neighbour’s building, work on shared walls, or building new walls on the boundary line. Your neighbour can either consent or request modifications. In some cases, a party wall surveyor will be needed to prepare a formal agreement.
Ignoring the Act can cause delays, legal disputes, and extra costs. Formi Architecture helps clients follow the correct notice process, ensures clear communication with neighbours, and resolves issues before work starts, keeping projects on track.
Cost of Applying for Planning Permission
In the UK, the standard application fee for a householder planning permission is currently £258 (2025 rate), but this is just one part of the total cost. You will likely need professional architectural drawings, planning statements, and possibly specialist reports such as flood risk assessments or heritage statements if your property is in a sensitive location. These can add several hundred to several thousand pounds depending on the project’s complexity.
Some councils also charge for pre-application advice, which can be useful for clarifying whether your extension is likely to be approved. While this adds to the initial outlay, it can prevent wasted time and money on a rejected application.
At Formi Architecture, we provide clear, itemised cost estimates at the start of the process. This includes the planning fee, professional fees, and any additional services so you know exactly what to budget for. By planning your finances early, you reduce the risk of delays caused by funding shortfalls partway through your project.
What Happens If You Build Without Planning Permission?
If you build an extension without the required planning permission, the local council can take enforcement action. This may include ordering you to submit a retrospective application, modify the extension, or even demolish it entirely. While retrospective planning permission is possible, it is not guaranteed, and the council can still refuse approval.
Building without permission can also create problems when selling your property, as solicitors and buyers will want proof that all works are compliant. In some cases, you may need to purchase indemnity insurance to reassure buyers, but this does not remove the risk of enforcement.
Formi Architecture helps homeowners avoid these issues by ensuring permissions are in place before work begins. If you already have an unapproved extension, we can assist with preparing a strong retrospective application and liaising with the council to find a practical resolution.
Summary: Planning Permission for Extensions in 2025
Understanding whether you need planning permission for an extension in 2025 depends on the type, size, and location of your project. Many small extensions fall under permitted development rights, but there are strict limits on height, depth, and proximity to boundaries. Special rules apply to conservation areas, listed buildings, and homes under Article 4 Directions.
You will almost always need building regulations approval, regardless of planning permission status, to ensure the extension meets safety and efficiency standards. Prior approval processes and neighbour consultations apply for larger permitted development projects. The Party Wall Act also needs consideration when working near shared boundaries.
At Formi Architecture, we combine architectural expertise with detailed knowledge of planning law to give you clear, actionable advice from the start. This approach reduces the risk of delays, avoids unnecessary costs, and ensures your extension project runs smoothly.
Frequently Asked Questions
Can I extend my house without planning permission?
Yes, if your extension meets permitted development criteria, but building regulations approval is still required.
How close can I build to my neighbour’s boundary?
Under permitted development, you can usually build right up to the boundary for certain single-storey extensions, but side extensions within 2 metres of the boundary have height restrictions.
What is the 3m and 6m rule for extensions?
Terraced and semi-detached homes can extend up to 3 metres to the rear under standard permitted development. Detached homes can extend up to 4 metres. Larger extensions up to 6 or 8 metres require prior approval.
Does an architect help with planning applications?
Yes. An architect can prepare compliant designs, submit applications, and manage communication with the council, greatly improving your approval chances.
Disclaimer
Planning permission requirements and costs can vary significantly depending on your location, property type, and local council policies. The information provided here is based on UK planning rules in 2025. Always consult a qualified architect or your local planning authority for advice specific to your project and area.
