Planning a house extension is an exciting opportunity to create more space, improve your home’s layout, and increase its functionality without moving to a new property. Whether you are considering a rear extension, side return, kitchen extension, or a larger home improvement project, understanding the potential costs is an essential first step.
But how much does a house extension cost in South London in 2026?
A house extension in South London can cost from tens of thousands of pounds to well over £150,000, depending on the size, design, structural requirements, specification, and level of finish. A well-specified single-storey rear extension may cost approximately £100,000–£150,000 for a typical 20–35 m² project, based on the indicative range published by Excel Construction for its South London projects. Actual costs depend on the individual property and scope of work.
In this guide, we explain the factors that influence house extension costs, the different types of extensions available, planning considerations, and how to prepare for a successful building project.
Quick Answer: How Much Does a House Extension Cost in South London?
The cost of a house extension depends on several factors, including the size of the extension, the existing structure of your property, materials, labour, architectural design, and internal finishes.
For budgeting purposes, London construction cost guides commonly publish figures in the region of £2,500–£5,000+ per square metre, depending on the project’s specification and what is included in the estimate. These figures should be treated as broad planning guidance rather than a fixed quotation.
Excel Construction’s indicative guide for a well-specified single-storey rear extension in South London is £100,000–£150,000 for a typical 20–35 m² project.
The final price may be higher or lower depending on the following:
- Size and layout of the extension
- Foundation and ground conditions
- Structural steelwork and alterations
- Roof design and glazing
- Kitchen, bathroom, and other installations
- Quality of materials and finishes
- Professional fees and approvals
- Access to the property and site conditions
The most reliable way to establish your actual budget is to arrange a property assessment and request a detailed quotation based on your requirements.
What Factors Affect the Cost of a House Extension?
Two homes can have extensions of a similar size but very different construction costs. The reason is that an extension is more than simply adding walls and a roof. It involves integrating a new structure with your existing property while meeting relevant building standards and your design expectations.
Here are the main factors to consider.
1. Size and Design of the Extension
The size of your extension is one of the main cost considerations, but the total area alone does not determine the price.
A straightforward rectangular extension may be less complex to construct than a design involving multiple roof levels, extensive glazing, unusual structural requirements, or difficult access.
Before starting, consider:
- How much additional floor space do you need?
- Will the extension be used as a kitchen, dining room, living area, or multiple rooms?
- Do you want an open-plan layout?
- Will the new extension connect to existing plumbing and electrical systems?
- Are you planning a single-storey or two-storey extension?
A clear design brief can help you and your contractor establish a more realistic budget.
2. Foundations and Groundworks
Foundations provide the structural support for your extension. The required foundation design depends on the ground conditions, existing buildings, proximity to neighbouring properties, and structural engineer’s recommendations.
South London properties may encounter different ground conditions, and some sites require additional investigation or more substantial foundation solutions.
Potential cost factors include:
- Soil conditions
- Foundation depth and design
- Drainage and sewer locations
- Proximity to neighbouring structures
- Excavation and removal of soil
- Structural requirements
Groundworks should be assessed during the planning stage. Unexpected site conditions can affect both the project budget and construction schedule.
3. Roof Design and Materials
Your roof specification can influence the appearance, energy performance, and overall cost of your extension.
Common options include flat roofs, pitched roofs, rooflights, and roof lanterns. The best choice depends on the property, design objectives, planning requirements, and budget.
For example, a roof incorporating large areas of glazing may require additional materials and installation work compared with a simpler roof design.
When discussing roof options, consider:
- Insulation and thermal performance
- Waterproofing and drainage
- Rooflights or lanterns
- Material durability
- Maintenance requirements
- Compatibility with the existing property
A suitable roof design should balance aesthetics, practicality, and long-term performance.
4. Structural Openings and External Walls
Many rear extensions involve removing part of the existing rear wall to connect the new space to the house.
This work may require temporary support, structural steelwork, new openings, brickwork, and making good the existing structure.
The final cost depends on the size of the opening, the structure of the existing building, and the design specified by the relevant professionals.
If you want an open-plan kitchen and dining area, discuss the structural implications early in the process. This helps ensure that the proposed layout is realistic and properly designed.
5. Internal Finishes and Fit-Out
Internal finishes can significantly influence the final cost of your house extension.
There is a substantial difference between a basic functional finish and a high-specification interior featuring premium flooring, bespoke cabinetry, architectural lighting, and custom joinery.
Your budget may need to account for:
- Flooring
- Plastering and painting
- Internal doors
- Joinery and storage
- Kitchen installation
- Bathroom fittings
- Lighting
- Decoration
If your extension includes a new kitchen, the cabinetry, worktops, appliances, and installation requirements should be considered separately within your overall project budget.
Types of House Extensions in South London
Choosing the right type of extension depends on your property, available space, lifestyle, and long-term plans.
1. Single-Storey Rear Extension
A rear extension is a popular option for homeowners who want to expand their kitchen, dining, or living space.
It can help improve the connection between indoor and outdoor areas while creating a more practical layout.
Potential benefits:
- Additional ground-floor living space
- Improved kitchen and dining layouts
- Better connection to the garden
- Opportunities for rooflights and larger glazing
- Greater flexibility in home design
The cost depends on the size, specification, structural work, and property conditions. Excel Construction’s indicative 2026 range for a well-specified single-storey rear extension is £100,000–£150,000 for a typical 20–35 m² project.
2. Side Return Extension
A side return extension uses the narrow side space often found beside terraced or semi-detached houses.
It can be particularly useful for expanding a kitchen or creating a more functional ground-floor layout.
However, side return projects may involve restricted access, drainage considerations, boundary constraints, and structural complexities.
Before committing to the design, assess how much usable space can realistically be created and whether the project requires planning permission or other approvals.
3. Wraparound Extension
A wraparound extension combines a rear extension with a side extension.
This option can create a substantial change to the ground-floor layout and may be suitable for homeowners who want to redesign several connected spaces.
Because of the larger footprint and potentially more complex structural and roofing work, a wraparound extension can require a higher budget than a straightforward rear extension.
The final cost depends on the design, property dimensions, site conditions, and level of finish.
4. Two-Storey Extension
A two-storey extension adds space across two levels and may be considered when a homeowner needs more floor area than a single-storey project can provide.
It can offer additional bedrooms, bathrooms, or living areas, but typically involves more extensive design, structural, and construction requirements.
Important considerations include:
- Foundation and structural design
- Roof integration
- Internal layout
- Planning requirements
- Building regulations
- Impact on neighbouring properties
A professional assessment is essential before establishing the feasibility and cost of a two-storey extension.
Do You Need Planning Permission for a House Extension in South London?
Planning permission requirements depend on the proposed extension, property type, location, dimensions, and applicable planning rules.
Some extensions may qualify for permitted development rights, while others require a planning application. Permitted development rights are subject to specific limitations and conditions, and they do not apply in every situation.
Factors that may affect your planning requirements include:
- Extension dimensions
- Height and position
- Property type
- Previous extensions
- Conservation area restrictions
- Listed building status
- Local planning policies
Do not assume that your extension is exempt from planning permission simply because it is small. Check the applicable requirements before construction begins.
You can review official guidance through the UK Government’s Planning Portal and consult the relevant local planning authority.
It is also important to distinguish planning permission from building regulations approval. These are separate requirements, and your project may need to comply with both.
How Much Should You Budget Before Starting?
A realistic budget should include more than the basic construction work.
Depending on your project, you may need to allow for:
- Design and architectural services
- Structural engineering
- Planning-related costs
- Building regulations compliance
- Construction and labour
- Kitchen and bathroom installations
- Fixtures, fittings, and finishes
- Professional inspections or other project costs
- Contingency for unforeseen work
The exact costs depend on your project scope and the professionals involved.
Example: Planning a 25 m² House Extension
Suppose you are considering a 25 m² single-storey extension.
Using an illustrative planning rate of £3,000 per square metre:
25 m² × £3,000 = £75,000
This is a mathematical illustration, not a quotation. It does not establish the actual cost of a South London project and may not include all professional fees, VAT, or high-specification finishes.
For a more realistic budget, obtain a detailed scope of work and quotation that clearly explains what is included.
A contractor’s quote should be reviewed carefully to establish whether it covers:
- Foundations
- Structural work
- Roofing
- Windows and doors
- Insulation
- Electrical and plumbing work
- Internal finishes
- Waste removal
- Project management
Comparing quotations on the basis of the same scope can help you understand the differences between contractors.
How Long Does a House Extension Take to Build?
The duration of a house extension depends on the size and complexity of the project, design readiness, approvals, site conditions, material availability, and construction arrangements.
A straightforward extension may take several months to complete, while larger or more complex projects can require a longer programme.
The overall process may include:
- Initial consultation
- Site assessment
- Design and structural planning
- Planning and building regulations processes, where applicable
- Final quotation and project preparation
- Groundworks and foundations
- Structural construction
- Roofing and external works
- Plumbing and electrical installation
- Internal finishes
- Final checks and handover
Ask your contractor for an indicative construction programme and clarify which stages are included.
Remember that approval timelines and construction timelines are separate. Delays in design decisions, materials, approvals, or unforeseen site conditions can affect the completion date.
How to Choose a House Extension Contractor in South London
Selecting a building contractor is an important decision because the quality of construction, communication, and project management can affect the entire experience.
Before appointing a contractor, consider the following.
Check Relevant Experience
Look for experience with projects similar to yours. A contractor who has completed rear extensions, kitchen extensions, or more complex residential renovations may be able to explain potential challenges more effectively.
Ask to see relevant project examples and discuss the scope of work.
Review the Quotation
A low initial quotation does not always represent the lowest overall project cost.
Make sure you understand:
- What work is included
- Which materials are specified
- Whether VAT is included
- How changes will be handled
- What is excluded
- Payment stages
- Expected project duration
A detailed quotation makes it easier to compare proposals.
Discuss Communication and Project Management
Ask who will manage the project, how progress will be communicated, and how any issues will be addressed.
Coordinating multiple trades and construction stages requires organisation and clear communication.
Check Insurance and Contract Arrangements
Before work begins, clarify the contractor’s insurance arrangements, written contract, responsibilities, and procedures for dealing with changes or defects.
The precise protections and requirements depend on the project and contractual terms.
Why Choose Excel Construction for Your South London Extension?
Excel Construction Ltd is an independent construction and development company serving South London and surrounding areas.
The company provides residential construction services, including:
- House extensions
- Loft conversions
- New builds
- Home renovations and related construction work
Excel Construction states that it has over 20 years of experience and has completed more than 1,000 projects. Its team provides an integrated approach to construction, coordinating multiple trades under one roof.
The company covers areas including Croydon, Wandsworth, Merton, Sutton, Lambeth, Southwark, Lewisham, Bromley, Bexley, Kingston upon Thames, and Richmond upon Thames.
Every extension project has different requirements. The best starting point is to discuss your property, proposed design, and budget so that the work can be assessed appropriately.
Frequently Asked Questions About House Extension Costs in South London
1. What is the average cost of a house extension in South London in 2026?
The cost varies according to size, specification, and structural requirements. Excel Construction’s indicative guide gives a range of £100,000–£150,000 for a well-specified 20–35 m² single-storey rear extension. This is a project-specific planning range, not a universal average for every extension.
2. Is it cheaper to build a rear extension or move house?
The financial decision depends on your property value, moving costs, available space, extension budget, and long-term needs. An extension may allow you to remain in a location you like, but you should compare the full costs and benefits of both options.
3. Can I build a house extension without planning permission?
Some projects may qualify for permitted development rights, subject to the relevant conditions and restrictions. Others require planning permission. Confirm the requirements for your specific property before construction.
4. How long does it take to build a rear extension?
The timeline depends on the size, complexity, approvals, materials, and site conditions. A contractor should provide an indicative programme based on your project.
5. What is the most expensive part of a house extension?
There is no single most expensive element for every project. Foundations, structural alterations, internal fit-out, glazing, and kitchen installations can all significantly influence the budget, depending on the design.
6. How can I avoid unexpected house extension costs?
Start with a clear design and detailed quotation. Investigate site conditions, clarify exclusions, understand the approval requirements, and discuss how changes or unforeseen work will be handled before construction begins.
7. Does Excel Construction provide house extensions in South London?
Yes. Excel Construction offers house extension services across South London and surrounding areas. Homeowners can contact the company to discuss their requirements and request a consultation.
Final Thoughts: Plan Your House Extension with Confidence
A house extension can be a valuable way to improve your home, create more usable space, and adapt your property to your family’s needs.
However, successful construction begins with realistic budgeting, thoughtful design, appropriate approvals, and a reliable contractor.
Whether you are planning a kitchen extension in Croydon, a rear extension in Wandsworth, or a larger home improvement project elsewhere in South London, take the time to understand your options before making a commitment.
Ready to discuss your house extension project?
Contact Excel Construction Ltd to arrange a consultation and explore the possibilities for your home.
Cost figures in this guide are indicative planning information, not fixed quotations. Actual costs depend on individual project requirements, property conditions, specification, and the scope of work.