A side extension can be an effective way to create additional space without taking too much from the garden. It can provide room for a larger kitchen, dining area, home office, utility room, or additional living accommodation, depending on the size and layout of the property. The cost of a side extension varies according to its size, number of storeys, construction method, materials, and specification. Structural alterations, foundations, glazing, roofing, professional fees, and internal finishes can all affect the final budget.
How Much Do Side Extensions Cost?
A typical side return extension in the UK currently costs around £30,000–£65,000, with a small single-storey side return averaging approximately £37,500. A small double-storey side return can cost around £45,000–£65,000, while current estimates place the average cost per square metre at around £3,000, with prices varying significantly by location and specification.
More generally, house side extensions are estimated at around £1,800–£3,000 per square metre. The final cost will depend on the size of the extension, the complexity of the design, the condition of the existing property and the level of finish required. These figures are indicative rather than fixed prices. A side extension with standard finishes may cost considerably less than a project involving structural glazing, a new kitchen, or extensive structural alterations.
What Does a Side Extension Cost?
Small Single-Storey Side Extension
A small single-storey side return extension can cost approximately £30,000–£45,000, with an average of around £37,500 based on current UK cost estimates. This type of extension can be suitable for widening an existing kitchen or living area, creating a utility space, or making better use of a narrow side passage.
Double-Storey Side Extension
A double-storey side extension generally costs more because it involves additional structural work, an upper floor, and more extensive roofing and construction. Current estimates for a small double-storey side return place the cost at around £45,000–£65,000, while a larger 5m × 4m double-storey side extension can cost around £66,000–£100,000, depending on specification.
Side Return Kitchen Extension
A side return is particularly useful for enlarging a kitchen and creating a more open kitchen-dining space. The extension itself may cost around £30,000–£45,000, while a new kitchen could add approximately £5,000–£14,000, depending on the specification. Premium cabinetry, appliances, worktops and specialist fittings can increase the kitchen budget considerably.
What Factors Affect Side Extension Costs?
Size and Number of Storeys
The size of the extension is one of the most significant factors affecting the overall cost. A larger extension requires more foundations, materials, labour, roofing, and internal finishes. Adding a second storey also increases the structural requirements and may require additional scaffolding and more complex construction.
Structural Work
Side extensions often involve removing part of an existing external wall to connect the new space to the house. Where the wall is load-bearing, structural calculations and steelwork may be required. The existing structure and condition of the property should therefore be assessed before construction begins.
Foundations and Ground Conditions
The required foundations depend on the proposed extension, existing building, and ground conditions. Unexpected ground conditions, nearby trees, drainage, or other site constraints can increase construction costs. Establishing the likely foundation requirements during the design stage can help create a more realistic budget.
Materials and Glazing
The choice of materials can have a significant impact on the overall cost. Standard brickwork may be more economical than extensive structural glazing or bespoke steel-framed systems. A glazed side return can cost around £3,000 per square metre, depending on the design and specification.
Kitchen and Internal Finishes
If the side extension is being used to create a new kitchen or dining area, the cost of cabinetry, appliances, flooring, lighting and other finishes needs to be included in the overall budget. Higher-specification finishes can increase the cost substantially beyond the basic construction price.
Does a Side Extension Need Planning Permission?
Some side extensions can be carried out under permitted development rights if the relevant limitations and conditions are satisfied. Under current permitted development rules in England, a side extension must be single-storey, must not exceed four metres in height and cannot be wider than half the width of the original house.
The rules also consider the original house and any previous extensions or other buildings on the land. Planning conditions, Article 4 Directions and other restrictions may remove or limit permitted development rights. Where the proposed extension falls outside the permitted development limits, a householder planning application may be required.
Can a Side Extension Be Permitted Development?
A side extension can qualify as permitted development where it meets the applicable requirements. In addition to the specific side extension restrictions, general conditions apply. For example, the extension cannot be higher than the highest part of the existing roof, and where it comes within two metres of a boundary, its eaves height cannot exceed three metres. Exterior materials should also have a similar appearance to those of the existing house. On certain designated land, including conservation areas, side extensions are subject to additional restrictions. Extensions on any side wall of the original house are not permitted development in these areas.
Can a Two-Storey Side Extension Be Permitted Development?
A two-storey side extension cannot normally benefit from permitted development rights where it extends beyond the side elevation of the original house. The permitted development rules specifically limit side extensions beyond the side elevation to single-storey development. A two-storey proposal will therefore generally require planning permission where it falls outside other permitted development provisions. The planning requirements should be established before developing detailed designs for a two-storey side extension.
Do Side Extensions Need Building Regulations Approval?
Planning permission and building regulations are separate requirements. A side extension will generally need to comply with building regulations, even where it is permitted development. The regulations cover matters including structural stability, foundations, insulation, ventilation, fire safety, drainage, electrical installations, windows and doors. Where the extension involves removing an existing external wall, structural calculations and appropriate construction details may also be required.
What Other Costs Should You Budget For?
The construction price is only part of the total project budget. Other costs may include:
- Architectural and planning drawings
- Structural engineering
- Planning application fees, where required
- Building Regulations approval
- Party Wall Matters
- Drainage alterations
- Kitchen or bathroom fittings
- Specialist glazing
- Landscaping and external works
- Professional consultant fees
- Contingency for unexpected costs
Party wall matters can be particularly relevant where the extension is close to or directly affects a shared boundary or party structure.
Common Side Extension Cost Mistakes to Avoid
Side extension projects can become more expensive when important requirements are overlooked. Common mistakes include:
- Budgeting only for the construction work.
- Underestimating foundation and structural requirements.
- Choosing high-specification glazing without allowing for the additional cost.
- Failing to budget for professional fees.
- Assuming permitted development rights automatically apply.
- Not checking previous extensions against current permitted development limits.
- Forgetting to include kitchen or bathroom costs.
- Starting work before confirming building regulations requirements.
- Failing to allow a contingency for unexpected work.
A detailed assessment and realistic budget can help reduce the risk of unexpected costs during construction.
Should You Get a Planning Appraisal for Your Side Extension?
A planning appraisal can help establish whether your proposed side extension is likely to benefit from permitted development rights or whether planning permission may be required. It can also identify restrictions affecting the property, including planning conditions, previous development, conservation area status, and other site-specific considerations. Where you intend to rely on permitted development rights, obtaining a Lawful Development Certificate may also provide formal confirmation that the proposed development is lawful.
How Studio Charrette Can Support Your Side Extension
Studio Charrette can support homeowners with the planning and architectural aspects of side extension projects, from assessing the initial proposal through to drawings, planning applications, and building regulations. The design can be developed around the existing property, the intended use of the additional space, and the relevant planning requirements. Professional advice at an early stage can help you understand the planning route, identify potential constraints, and establish a realistic budget before construction begins.
Planning a Successful Side Extension
The cost of a side extension depends on its size, number of storeys, structural requirements, materials, glazing and internal specification. Current UK estimates place a typical side return extension at around £30,000–£65,000, while more complex or higher-specification projects can cost considerably more.
Before starting work, it is important to consider the construction cost alongside professional fees, planning requirements, building regulations, and any additional structural or site-related costs. A clear understanding of the project requirements can help you make informed decisions about the size and specification of your extension. If you need advice on your proposed side extension, contact Studio Charrette for professional planning and architectural support.
Frequently Asked Questions
How much does a side extension cost in the UK?
A typical side return extension currently costs around £30,000–£65,000. A small single-storey side return averages approximately £37,500, although the final cost depends on the size, location, and specification.
Is £30,000 enough for a side extension?
£30,000 may be sufficient for a small, straightforward single-storey side extension using standard materials, particularly outside London. Larger extensions, premium finishes, and more complex structural work can increase the budget considerably.
Can I build a side extension without planning permission?
Some single-storey side extensions can be permitted development if they comply with the relevant limitations and conditions. The extension must generally be no more than four metres high and no wider than half the width of the original house.
Can I build a two-storey side extension under permitted development?
A side extension beyond the side elevation of the original house must be single-storey to qualify under the relevant permitted development rules. A two-storey side extension will generally require planning permission.
Do I need building regulations for a side extension?
Building regulations requirements generally apply to side extensions, even where the extension does not require planning permission.
Is a side extension cheaper than a rear extension?
There is no fixed price difference. The cost depends on the size, design, construction method, specification, and structural requirements of each project. Current general estimates for side extensions are around £1,800–£3,000 per square metre.
