Basement Extension Guide: Costs, Planning & Feasibility 2026
A basement extension creates living space beneath your home. It is the most expensive per-square-metre option but may be the only choice when you cannot extend outwards (terraces, conservation areas, tight plots). Basement projects require specialist contractors and careful engineering.
What you need to know
Conversion vs new basement
If your home already has a cellar, converting it to habitable space is significantly cheaper (£1,500–£2,500/m²) than digging a new basement (£3,000–£5,000/m²). Cellar conversions involve waterproofing, insulating, and fitting out an existing space. New basements require full excavation beneath the existing structure.
Structural considerations
Digging out a basement beneath an existing house is complex engineering. The existing foundations must be underpinned (supported while the ground beneath is excavated). The new basement walls must resist water pressure and soil loads. This is specialist work — not every builder can do it.
When a basement makes sense
Basements are worth considering when: you have no garden space to extend into, your property is in a conservation area where external changes are restricted, you need a cinema room, gym, or utility space that does not need natural light, or you live in an area where property values are high enough to justify the cost.
How much does it cost?
| Type | Typical Cost Range |
|---|---|
| Cellar conversion (per m²) | £1,500–£2,500 |
| New basement (per m²) | £3,000–£5,000 |
| Typical cellar conversion (20m²) | £30,000–£50,000 |
| Typical new basement (30m²) | £90,000–£150,000 |
| Light well installation | £5,000–£15,000 |
| Waterproofing system | £5,000–£15,000 |
| Professional fees | £5,000–£15,000 |
Prices based on Dorset rates, 2026
Planning permission
Cellar conversions (within the existing footprint) usually fall under Permitted Development. New basements that extend beyond the footprint require planning permission. Light wells and external access may also need planning.
Internal alterations (including converting an existing cellar) are typically not controlled by PD. Excavation beneath the existing footprint may also be PD. Any extension beyond the existing footprint requires a planning application.
Building regulations
Building Regulations for basements are extensive: structural calculations for underpinning and basement walls, waterproofing to BS 8102, adequate ventilation and drainage, fire safety (escape route to ground level), and minimum ceiling height of 2.1m.
Typical timeline
| Phase | Duration |
|---|---|
| Feasibility study | 2–4 weeks |
| Design and planning | 4–12 weeks |
| Underpinning and excavation | 6–12 weeks |
| Waterproofing and structure | 4–6 weeks |
| Fit-out | 4–8 weeks |
Frequently asked questions
Ready to get started?
Professional design plans for your project. Fixed-price, no obligation.
Every plan drawn by qualified drafters in Poole, Dorset
Serving homeowners across Poole, Bournemouth, Christchurch, Wimborne, Wareham, and the wider Dorset area.