If you are planning an extension, one of the first questions is:
How much do extension costs in London typically come to in 2026?
While exact construction prices depend on contractors, scope and specification, there are consistent cost drivers that influence pricing across London projects.
Understanding these factors helps you define the right scope before starting architectural drawings.
Below is a breakdown of typical London cost ranges — and why they vary.
Single-Storey Extension Cost (London 2026)
£2,500–£3,500 per m²
Single-storey rear extensions are the most common type of extension in London. However, the final cost per m² depends on several technical factors.
What drives the cost?
1. Foundations and ground conditions
London clay often requires deeper foundations. If nearby trees influence the soil, foundations may need to go deeper, increasing excavation and concrete costs.
2. Structural openings
Most single-storey extensions involve removing the rear wall to create open-plan space. This requires steel beams and temporary structural support, adding cost.
3. Glazing specification
Large aluminium sliding doors, rooflights and bespoke glazing significantly increase cost compared to standard uPVC windows.
4. Access constraints
Many London properties have restricted side access. Limited access increases labour time and sometimes requires materials to be brought through the house.
5. Finish level
Floor finishes, underfloor heating and kitchen integration all affect overall cost.
Side-return extensions often sit at the higher end due to structural complexity and tight construction conditions.
Two-Storey Extension Cost (London)
Additional storey: £1,050–£1,500 per m². Added to the rear extension cost.
Two-storey extensions often appear cheaper per square metre than single-storey builds.
Why?
Because:
- The cost of foundations is shared across two floors.
- The roof cost is proportionally lower per m².
- Scaffolding is already required for upper-level work.
However, structural complexity increases. Load paths must be carefully designed, and integration with the existing first floor can require more intrusive works.
Planning considerations are also stricter for two-storey extensions due to overlooking and scale impact.
Loft Conversion Cost in London
£1,350 – £2,500 per m²
Loft conversions can be cost-efficient because they avoid major groundwork. However, they are structurally demanding.
Key cost factors:
1. Existing roof structure
Traditional cut roofs are easier to convert than modern trussed roofs, which require more structural alteration.
2. Structural floor upgrades
Existing ceiling joists are not designed as floor joists. A new structural floor system is required.
3. Staircase insertion
The staircase location affects layout and complexity.
4. Dormers and roof alterations
Rear dormers increase head height but add construction and waterproofing costs.
5. Fire regulations upgrades
Loft conversions trigger fire safety upgrades, including fire doors and protected escape routes.
Basement Extension Cost (London)
£3,000–£5,000 per m² (under existing house)
£3,000–£4,000 per m² (under garden)
Basements are among the most complex residential projects in London.
Why are they expensive?
1. Excavation and soil removal
Excavation often requires careful sequencing, particularly in terraced houses.
2. Underpinning
Existing foundations must be supported during excavation.
3. Waterproofing systems
Basements require cavity drainage membranes or tanking systems to comply with building regulations.
4. Structural engineering
Temporary works design is critical.
5. Neighbour impact
Party Wall matters and neighbour protection can increase complexity.
Access is often the single biggest cost driver.
Structural Openings (Steel Beams)
£3,000–£10,000 per opening
Creating open-plan layouts requires:
- Structural calculations
- Temporary propping
- Installation of steel beams
- Building control inspection
Longer spans require heavier steel sections. Tight sites increase labour time.
Chimney Removal Cost
£2,000–£5,000 per chimney
Removal requires:
- Structural support
- Careful dismantling
- Making good finishes
- Potential roof work
Shared stacks require neighbour coordination.
To consider
The cost will increase if your existing structure is complex or if you are building a completely new extension. If the current roof and proposed layout are complicated, it may be more practical to remove the existing structure and install a new roof instead.
Why This Matters Before Starting Drawings
Architectural drawings define:
- Size
- Structure
- Glazing strategy
- Layout
- Compliance with planning policy
Understanding realistic London cost drivers helps you:
- Define the correct extension size
- Select the right extension type
- Align expectations with likely construction realities
With the right scope from the start, your planning and building regulations drawings can be developed efficiently and confidently.
Start With a Free Extension Feasibility Check
Not ready to commit to full drawings?
Discover if your extension ideas are likely to receive planning approval before investing time and money.
Provide your details and we will be in touch to discuss your project.
During this initial conversation, we will:
- Review your property location
- Discuss your extension proposal
- Highlight planning considerations
- Share realistic London cost-per-m² ranges
- Explain next steps for drawings
👉 Request your free feasibility check here
No obligation. Just clarity before moving forward.