The short answer
Building control approval is required for virtually every basement conversion, while planning permission is only needed in some cases. A householder planning application in England carries a fixed government fee in the low hundreds of pounds, and is typically needed where you create a lightwell on a visible elevation, add a separate entrance, change the use, or work on a listed building or in a conservation area. Building control fees usually run from around £500 to £1,500+ depending on the project, and cover checking the structure, waterproofing, fire escape and ventilation against the Building Regulations. Converting an existing cellar internally often needs only building control, not planning, but a new dig or external alteration frequently needs both. Always check with your local authority before assuming.
Statutory fees are a small share of a basement budget but an essential one, and skipping the approvals is never an option. The sections below set out what planning and building control cost and when each applies.
At a glance
- Householder planning feeFixed gov fee, low hundreds
- Building control~£500–£1,500+
- Building controlRequired almost always
- PlanningSometimes, depends on works
- Listed / conservationMore likely to need planning
When you need planning permission
Many internal cellar conversions do not need planning permission, because converting existing space below a house is often treated as internal alteration. Permission becomes more likely when the work changes the building's appearance or use. Common triggers include forming a lightwell or new external entrance on a visible elevation, creating a self-contained flat (a change in the number of dwellings), or working on a listed building or within a conservation area, where even modest external changes are scrutinised. Some councils also have specific basement policies, particularly in parts of London, that control the size and depth of new basements. The government planning fee for a householder application is fixed and modest, but the design and drawing costs to support it add to the overall figure.
| Type of work | Planning likely? | Building control? |
|---|---|---|
| Internal cellar conversion | Often no | Yes |
| New lightwell (visible) | Often yes | Yes |
| Separate external entrance | Usually yes | Yes |
| New basement dig | Often yes | Yes |
| Listed / conservation area | Usually yes | Yes |
Indicative guidance only. Always confirm with your local planning authority.
What building control covers
Building control approval is needed for almost every basement, because the work engages so many of the Building Regulations. The building control body, whether the local authority or an approved inspector, checks the structure (the underpinning, retaining walls and slab against the engineer's design), the waterproofing, the means of escape and fire safety for a habitable room, and the ventilation. Inspections happen at key stages, and the project is signed off with a completion certificate that you will need when you come to sell. The fee depends on the scale of the work and the body you use, typically a few hundred to well over a thousand pounds.
How these fees fit the budget
Planning and building control fees are a small part of a basement budget compared with the structural work, but they are real and should be allowed for. Beyond the government planning fee and the building control charge, there is the cost of the drawings and supporting information an application needs, often prepared by an architect or designer, which can exceed the statutory fees themselves. On a basement, the building control submission also relies on the structural engineer's calculations, so these professional inputs link together.
The sensible approach is to establish early, ideally with a pre-application enquiry to the council where there is any doubt, whether planning permission is required, and to confirm which building control route you will use. That avoids the twin risks of starting work that needs permission without it, or under-budgeting for the design and approval process. Because the consequences of skipping approvals, enforcement action or an unsellable basement, are serious, these fees are a necessary and modest insurance against far larger problems later.
Frequently asked questions
Does converting a cellar always need planning permission?
Not always. A purely internal cellar conversion is often classed as alteration and may not need planning permission, though building control approval is still required. Permission is more likely if you add a lightwell, external door, change the use, or the property is listed or in a conservation area.
What does building control check on a basement?
The structure (underpinning, retaining walls and slab), the waterproofing, the means of escape and fire safety for a habitable room, and the ventilation. Inspections happen at key stages and the work is signed off with a completion certificate you will need when selling.
What happens if I skip building control?
The work has no completion certificate, which can cause real problems when you sell, as buyers' solicitors ask for it. You may also have to open up finished work for inspection or apply for regularisation. Building control approval is not something to skip on a basement.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.