The short answer
It depends on whether you are converting an existing cellar or excavating a new basement. Simply changing the use of an existing below-ground space — without altering the external appearance and without a new lightwell or external access — is often permitted development and needs no planning application. By contrast, digging a new basement, lowering an existing floor, or forming a new lightwell, separate entrance or window almost always needs full planning permission, because it involves engineering works and external change. Flats, listed buildings and properties in conservation areas, National Parks or Areas of Outstanding Natural Beauty usually have permitted development restricted or removed, so a flat conversion or a listed home will need permission and possibly listed building consent. Some councils, especially in London, run specific basement planning policies. Always check with your local planning authority and the Planning Portal before you start, because the answer turns on your exact property and works.
The planning answer hinges on one distinction: are you re-using a space that already exists, or creating new below-ground space? The two are treated very differently. The notes below set out where each falls.
Planning at a glance
- Convert existing cellar (no external change)often permitted development
- Excavate a new basementusually full planning permission
- New lightwell / separate entranceneeds planning permission
- Flatspermitted development does not apply
- Listed / conservation areapermission + possible consent
When permitted development applies
Permitted development rights let you carry out certain works to a house without a planning application. Converting an existing cellar or basement from storage into a habitable room can fall within these rights, because it is treated as a change of use of existing space rather than an enlargement of the building. The key conditions are that you do not alter the external appearance, do not significantly change the structure of the building, and do not form a new lightwell, external staircase or separate entrance. As soon as the works break the surface outside — a new window well, a sunken patio, a lowered external floor or excavation beyond the existing footprint — they generally fall outside permitted development and need planning permission.
When you need full planning permission
Most genuine basement creation projects need planning permission. The trigger is usually one or more of the following: excavating new below-ground space, lowering an existing basement floor to gain head height, forming a new lightwell or external access, or any works that change the external appearance. London boroughs in particular have detailed basement policies covering the depth and extent of excavation, the proportion of the garden that can be dug, structural method statements, flood risk and the impact on neighbours, trees and groundwater. A flat conversion is treated as alteration to a flat, which is outside permitted development entirely, so it needs permission regardless of whether the space already exists.
| Situation | Planning position |
|---|---|
| Convert existing cellar, no external change | often permitted development |
| Excavate a brand-new basement | full planning permission |
| Lower an existing floor for head height | usually planning permission |
| Add a lightwell, window well or entrance | planning permission |
| Property is a flat / maisonette | planning permission (no PD) |
| Listed building | permission + listed building consent |
Indicative UK positions for guidance. Source: Planning Portal and local planning authority basement policies.
Listed buildings, conservation areas and local policy
If your home is listed, you will almost certainly need listed building consent for basement works in addition to any planning permission, and consent can be hard to obtain where excavation would affect historic fabric or foundations. In a conservation area, permitted development is curtailed and the council pays close attention to external changes such as lightwells and railings. Many authorities — particularly in London — have adopted specific basement development plans or supplementary planning documents that limit how deep and how far you can dig, require a structural method statement and a basement impact assessment, and protect neighbouring properties, trees and groundwater flow. A pre-application enquiry with your local planning authority is the reliable way to find out what applies, because two near-identical houses can be treated differently depending on their planning history and local policy.
Frequently asked questions
Can I convert my cellar without planning permission?
Often yes, if you are changing the use of an existing cellar and not altering the external appearance or adding a lightwell or separate entrance. Flats, listed buildings and conservation-area homes are exceptions where permission is usually required. Building Regulations approval is still needed either way.
Does digging a new basement always need planning permission?
In nearly all cases, yes. Excavating new below-ground space, lowering a floor or forming external works is engineering and external change, which falls outside permitted development. Many London boroughs also have specific basement policies governing depth, extent and neighbour impact.
Do I need planning permission for a basement under a flat?
Yes. Permitted development rights do not apply to flats and maisonettes, so any basement conversion or creation under a flat needs full planning permission, and the freeholder's consent and any lease terms must also be addressed.
Sources & further reading
- Planning Portal — do you need permission?
- Planning Portal — basements and cellar conversions
- GOV.UK — when is permission required
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.