Planning a house extension is one of the most effective ways to add value and space to your property. But before you start, it's essential to understand the costs involved in 2026.
Single Storey Extension Costs
A single storey rear or side extension is typically the most cost-effective way to add space to your home. In 2026, you can expect to pay between £30,000 and £55,000 for a quality single storey extension.
This price range covers a standard rear extension of around 20-30 square metres, including foundations, structural work, windows, doors, and internal finishes. Premium finishes or larger extensions will push costs towards the higher end.
Double Storey Extension Costs
Adding a double storey extension effectively doubles your floor space while the per-square-metre cost is lower than a single storey build. Expect to pay between £55,000 and £90,000 for a double storey extension.
The additional structural requirements for supporting the upper floor, including steel beams and foundation reinforcement, contribute to the overall cost. However, you're getting roughly twice the space for around 60-70% of the cost of two single storey extensions.
Side Return Extension Costs
Side return extensions utilise the narrow strip of land often found alongside terraced and semi-detached properties. These typically cost between £20,000 and £40,000.
While more straightforward structurally, side returns often require careful planning to maximise the available space and may involve removing supporting walls to create an open-plan kitchen-diner layout.
Factors Affecting Extension Costs
- Location: London and the South East typically see costs 20-30% higher than the North of England
- Ground conditions: Difficult ground, sloping sites, or poor soil can increase foundation costs significantly
- Structural complexity: Open-plan designs, large openings, and cantilever features require additional steelwork
- Finishes: Premium kitchens, bathrooms, and flooring will add significantly to the overall cost
- Planning requirements: If full planning permission is required, architect fees and application costs apply
- Existing structure: Removing existing structures or working around live buildings adds complexity
Additional Costs to Consider
Beyond the main build, remember to budget for:
- Architectural drawings and structural calculations: £2,000-£5,000
- Planning permission fees: £206 for full application
- Building regulations approval: £200-£500
- Party wall agreements (if applicable): £700-£1,500 per neighbour
- Utility connections and amendments: £1,000-£3,000
- Landscaping and external works: £3,000-£10,000
Getting an Accurate Quote
The only way to get an accurate cost for your specific project is to speak with experienced builders who can assess your property and requirements. At Doherty Builders, we provide free, detailed quotes with full cost breakdowns.
Contact us today to discuss your extension project and receive a tailored quote based on your specific requirements and property.