Extending your home is often a more cost-effective alternative to moving, allowing you to create additional living space while increasing the value of your property. However, before starting your project, it’s important to understand the true cost of a house extension and the factors that can influence your budget.
This guide explains the typical costs of different types of extensions in 2026, the hidden expenses many homeowners overlook, and practical ways to keep your project on budget.
Typical House Extension Costs
Construction costs vary depending on the size, specification, complexity and location of your project.
1. Single-Storey Extensions
A typical single-storey extension costs between £2,400 and £3,400 per m² plus VAT.
- Basic specification: £2,400–£2,800/m²
- Mid-range specification: £2,800–£3,200/m²
- High-end specification: £3,200–£3,400+/m²
A 30m² kitchen extension will typically cost between £72,000 and £102,000 plus VAT.
2. Two-Storey Extensions
Two-storey extensions generally cost £2,200–£3,200 per m² plus VAT.
Although the total project cost is higher, they usually provide much better value because the foundations and roof are shared across two floors.
3. Side Return Extensions
Side return extensions are generally more expensive due to restricted access and more complex construction.
Typical cost: £2,800–£3,800 per m² plus VAT
4. Conservatory and Garden Room Extensions
Costs vary depending on the specification.
Where Does Your Money Go?
A typical extension budget is broadly divided into three areas.
Structure and External Shell (Approximately 60%)
- Foundations
- Drainage
- External walls
- Structural steelwork
- Roof construction
- Windows and external doors
Internal Fit-Out (Approximately 30%)
- Electrical installation
- Plumbing
- Heating
- Plastering
- Flooring
- Decorating
- Kitchen or bathroom installation
Professional Fees (Approximately 10%)
- Architectural drawings
- Structural engineer
- Building Control fees
- Planning application fees (where required)
- Party Wall Surveyor (if applicable)
What Affects Extension Costs?
Location
Construction costs in London and the South East are often 20–35% higher than many other parts of the UK due to increased labour and material costs.
Specification
Premium glazing, bespoke joinery, rooflights, underfloor heating and luxury finishes can substantially increase the overall budget.
Site Conditions
Restricted access, difficult ground conditions, mature trees, drainage diversions or poor soil can all add unexpected costs.
Complexity
Simple rectangular extensions are generally much cheaper than designs incorporating multiple roof junctions, curves, large cantilevers or extensive glazing.
Construction Methods
Traditional Brick and Block
- Most common in the UK
- Durable and familiar
- Usually the most economical option
Timber Frame
- Faster construction
- Excellent insulation
- Lower embodied carbon
Structural Insulated Panels (SIPs)
- Excellent thermal performance
- Rapid installation
- Higher initial material cost
Insulated Concrete Formwork (ICF)
- Outstanding insulation
- Exceptional strength
- Popular for low‑energy homes
Additional Costs to Budget For
- VAT (usually 20%)
- Building Control fees (£600–£1,200)
- Structural engineer
- Party Wall Surveyor
- Thames Water Build Over Agreement (where applicable)
- Drainage surveys
- Tree reports
- Temporary accommodation
- Landscaping and external works
- Utility upgrades
A contingency of at least 10% should always be included to cover unforeseen costs.
Professional Fees
- Architectural designer or architect: 3–10% of construction costs
- Structural engineer: project dependent
- Quantity Surveyor: 1–3% of project value
Estimates vs Quotations
An estimate is an approximate indication of likely costs and may change as the design develops.
A quotation provides a detailed breakdown of the agreed scope of works and should clearly identify what is and is not included.
Always compare quotations on a like‑for‑like basis rather than simply choosing the cheapest price.
How to Reduce Extension Costs
- Keep the design simple and rectangular.
- Avoid unnecessary structural alterations.
- Use standard‑sized windows and doors.
- Minimise drainage diversions.
- Finalise the design before construction begins.
- Obtain detailed quotations from several reputable contractors.
Will an Extension Add Value?
A well‑designed extension can significantly increase the value of your property. Kitchen extensions commonly add 5–15% to a home’s value, while larger two‑storey extensions may add even more where there is strong local demand for additional living space.
However, every area has a ceiling value, so it’s important to ensure the cost of the extension is proportionate to the likely increase in value.
Is an Extension Worth It?
For many homeowners, extending offers better value than moving. It avoids estate agent fees, Stamp Duty Land Tax, legal costs and removal expenses while allowing you to create a home tailored to your needs.
Seeking professional advice at an early stage can also help identify opportunities to reduce costs without compromising quality.
Conclusion
House extension costs vary considerably depending on the size, specification, location and complexity of the project. Careful planning, realistic budgeting and detailed drawings are essential to achieving a successful outcome.
Before approaching builders, ensure your design has been properly developed and fully coordinated. This reduces the likelihood of unexpected costs during construction and allows contractors to provide accurate quotations.
If you’re thinking about extending your home, contact Draw & Plan for expert advice on planning applications, Building Regulations and construction drawings.
Disclaimer: This article is intended for general informational purposes only and does not constitute legal, structural or professional advice. Construction costs vary by location, specification and market conditions. Always obtain quotations from suitably qualified professionals before commencing any building work.