Side Extension Planning Permission UK

Side extension planning permission in the UK depends on the size, height, position and location of the proposed extension. Some side extensions can fall under permitted development, but a side extension is generally limited to one storey, up to 4 metres high and no wider than half the original house under the relevant permitted development rules. Protected locations can have additional restrictions.
What is side extension planning permission?
A side extension adds new space to the side of an existing house. Homeowners commonly use side extensions to enlarge kitchens, create utility rooms, add home offices or improve the layout of semi-detached and detached properties.
The important point is that planning permission and permitted development are not the same thing. A project may qualify for permitted development but still need Building Regulations approval, structural calculations or other legal requirements.
For homeowners in Hounslow, Middlesex and West London, checking the property’s planning history and local restrictions before finalising drawings can prevent expensive redesigns.
Do I need planning permission for a side extension?
Not always. A side extension may be possible under permitted development rights if it meets all applicable conditions.
For a typical house in England, permitted development rules for side extensions include these important limits:
- Maximum height: 4 metres
- Maximum width: half the width of the original house
- The extension must have only one storey
- Additional restrictions apply if the extension is within 2 metres of a boundary
- The development must not extend beyond certain parts of the original dwelling
- The total area covered by buildings within the property’s curtilage, excluding the original house, generally cannot exceed 50% of the curtilage
- Properties in certain protected areas can face stricter rules.
Because permitted development conditions must all be satisfied, measuring the property correctly is essential.
When does a side extension require full planning permission?
You are more likely to need a planning application if the proposed extension falls outside permitted development limits.
Examples include:
- The extension is more than half the width of the original house
- It is more than one storey
- It exceeds the permitted height limits
- The design extends beyond restricted parts of the original dwelling
- The property is in a conservation area or another specially protected location
- Permitted development rights have been removed or restricted
- The property is a flat rather than a house
The government’s technical guidance also confirms that permitted development rights do not remove requirements under other regimes, including Building Regulations and the Party Wall Act.
For a Hounslow property, it is therefore worth checking the specific site rather than assuming that a neighbouring extension automatically means your project will receive the same treatment.
What are the planning rules for side extensions?
Side extensions have several technical restrictions that homeowners often overlook.
Under permitted development rules, a side extension must generally be single-storey, no more than 4 metres high, and no wider than half the width of the original dwellinghouse. If an extension is close to a boundary, the eaves-height restriction can also become important.
A side extension that wraps around the rear of a property can also be affected by the rules applying to rear extensions. This means the dimensions of the entire proposal need to be considered together, particularly where an existing extension is already present.
How long does planning permission take?
For a standard householder planning application in England, the statutory decision period is 8 weeks once the application has been validated. The period can be extended if the applicant and local planning authority agree to a longer timeframe.
The latest government statistics show that 93% of householder applications were decided within 8 weeks or the agreed period in the year ending December 2025.
A practical planning timeline can include:
1. Initial consultation and site assessment
2. Measured survey and architectural design
3. Planning drawings and supporting information
4. Application submission
5. Validation and consultation
6. Planning decision — normally within 8 weeks
7. Building Regulations and construction preparation
The design and preparation period before submission is not included in the council’s 8-week statutory decision period.
How much does side extension planning permission cost?
As of 1 April 2026, the standard planning application fee in England for enlarging, improving or altering an existing single dwellinghouse is £548.
Your overall planning budget can also include professional costs for:
- Measured surveys
- Architectural drawings
- Planning application preparation
- Structural calculations
- Building Regulations drawings
- Specialist reports where required
- Council or consultant fees for additional services
The £548 planning fee is therefore not the same as the total cost of preparing a side extension project.

What Building Regulations apply to a side extension?
Planning permission controls whether development is acceptable from a planning perspective. Building Regulations deal with how the extension is designed and constructed safely.
Depending on the project, Building Regulations can cover:
- Structural stability and foundations
- Fire safety
- Thermal insulation and energy efficiency
- Ventilation
- Drainage
- Glazing and safety
- Electrical installations
- Accessibility and other technical requirements
Electrical work should comply with the relevant UK requirements, including Part P where applicable. Where gas work is included, use an appropriately registered Gas Safe engineer.
ERD Ltd provides planning, architecture and construction services from its base in Hounslow, Middlesex, allowing homeowners to coordinate design and construction requirements as one project. ERD’s published service information includes planning support, architectural design, extensions, construction and specialist home improvement services.
What can affect side extension planning approval?
Planning officers assess the proposal against relevant planning policies and consider factors such as the size, layout, siting and appearance of the development and its effect on the surrounding area.
For a side extension, important design considerations include:
- Overall height and width
- Relationship with neighbouring properties
- Impact on daylight and privacy
- Position of windows
- Roof design
- Materials and external appearance
- Parking and access
- Relationship with the existing house
- Local planning policies and restrictions
A well-designed extension should look like a logical addition to the property rather than an oversized structure squeezed into the side passage.
How can ERD Ltd help with side extension planning in Hounslow?
ERD Ltd provides planning and architecture services in Hounslow and West London, including architectural design, planning support, renovation and extension projects. The company states that its service covers the process from concept development through planning approvals and project management.
This can be particularly useful when a side extension involves several stages — for example, architectural design followed by structural work, electrical installation, kitchen renovation and internal finishing.
ERD Ltd is based at 205 Wellington Road South, Hounslow, Middlesex, and works with homeowners across Hounslow and West London.
Rather than relying on a generic permitted-development checklist, homeowners should have the actual property assessed before deciding whether a planning application is required.
Side Extension Planning Permission Checklist
Before starting your project, check:
- Is the property a house rather than a flat?
- Are permitted development rights available?
- Is the proposed side extension single-storey?
- Is it within the 4-metre height limit?
- Is its width no more than half the original house width?
- Is the property in a conservation area or another protected location?
- Are there previous planning conditions or restrictions?
- Will Building Regulations approval be required?
- Are structural calculations needed?
- Could the Party Wall Act apply?
- Are electrical or gas works included?
FAQ: Side Extension Planning Permission UK
Do I need planning permission for a side extension in the UK?
Not necessarily. A side extension may qualify for permitted development if it meets all applicable conditions, including limits on height, width and storeys. Otherwise, a full planning application may be required.
How wide can a side extension be without planning permission?
Under the standard permitted development rules, a side extension must generally be no wider than half the width of the original dwellinghouse. Other restrictions also apply, so width alone does not determine whether planning permission is required.
How high can a side extension be without planning permission?
A permitted-development side extension must generally be single-storey and no more than 4 metres high. Additional restrictions can apply depending on its distance from the property boundary.
How long does side extension planning permission take?
A standard householder planning application in England normally has an 8-week statutory decision period after validation. More complex applications or agreed extensions of time can take longer.
How much does planning permission cost for a side extension?
For a typical householder application in England, the planning application fee is £548 from 1 April 2026. Professional design, survey and technical costs are additional.
Do I need Building Regulations approval if my side extension does not need planning permission?
Yes, potentially. Permitted development only concerns planning permission; it does not remove Building Regulations requirements. The technical requirements depend on the construction and services included in the extension.