Plans to convert an existing cellar into living space in London start at £2,265 inc survey with us, or £2,065 if you can supply measured drawings. The price covers the survey, drawings, structural input and a check of your borough's basement policy. Lowering the floor or digging a new basement is quoted individually.
The difference matters in law as well as in cost. Works that only affect the inside of a building are, by statute, not development. But a basement dug to give additional space underground is. So a damp cellar made habitable may need no planning permission, while an excavation for a new floor does. Building regulations and the Party Wall Act apply to both.
Existing cellar or new excavation
The £2,265 inc survey is for a cellar that already exists under the house: we survey it, draw the conversion, and bring in structural input for any new openings and the stair. It includes a check of the borough's basement policy. If the job involves lowering the floor to gain head height, or digging out a new basement, the structure is a different scale of work and we quote it individually after a visit.
When planning permission is needed
The starting point is section 55 of the Town and Country Planning Act 1990. Works for the alteration of a building that affect only its interior are not development. But the same subsection takes out of that exemption works begun after 5 December 1968 for the alteration of a building by providing additional space in it underground. Digging out new space therefore needs permission, and fitting out an existing cellar does not.
Local policy can still bite. Boroughs with many basement schemes publish their own policies on depth, garden coverage and impact assessments, and some use an Article 4 direction. The cellar conversion may also change windows, lightwells or railings at the front, and external changes can need permission. That is why our price includes the policy check before we draw.
Photo: Ruth Sharville, BY-SA 2.0
Building regulations: where most of the cost sits
A cellar was built to store coal, not to live in. To become a bedroom or a study it must be dry, ventilated, lit, and have a way out in a fire. Building control needs to see a drawn solution for each, and a full-plans application is decided within 5 weeks, or 2 months if you agree.
- Damp: a waterproofing system, usually tanking or a cavity drain, designed for the site's groundwater.
- Headroom and stairs: the stair, landing and any floor lowering needed to reach a usable ceiling height.
- Escape: a route out or a protected stair, and a window that can be used as an escape from a bedroom.
- Ventilation and light: background ventilation, extraction, and daylight from lightwells or rooflights.
- Structure: new openings through load-bearing walls, and underpinning if the floor is lowered.
Digging near the neighbours
Underpinning and excavation bring the Party Wall etc. Act into play. Section 6 applies where you excavate within three metres of a neighbouring building to a depth below the neighbour's foundations, or within six metres where the excavation meets a 45 degree line from the neighbour's foundations. The building owner must serve notice at least one month before starting.
The figures in this article, and where they come from
Works that affect only the interior of a building are not development, except works begun after 5 December 1968 that provide additional space underground (section 55(2)(a)).
Source: legislation.gov.uk, Town and Country Planning Act 1990, section 55Section 6 of the Party Wall etc. Act 1996 covers excavation within 3 metres of a neighbouring building, or within 6 metres where it meets a 45 degree line from the neighbour's foundations.
Source: legislation.gov.uk, Party Wall etc. Act 1996, section 6The building owner must give notice at least 1 month before starting excavation under section 6 of the Party Wall etc. Act 1996.
Source: legislation.gov.uk, Party Wall etc. Act 1996, section 6(5)A full-plans building regulations application gets a decision within 5 weeks, or 2 months with the applicant's consent.
Source: GOV.UK, Building regulations approvalYou will usually get a completion certificate within 8 weeks of finishing the building work, as long as it complies.
Source: GOV.UK, Building regulations approvalRequirement A1 of the Building Regulations 2010 says loads must be sustained and transmitted to the ground safely.
Source: legislation.gov.uk, Building Regulations 2010, Schedule 1, requirement A1
Questions people ask
Do I need planning permission to convert my cellar?
Often not, if the work is internal and creates no additional space underground. Changes to the front, such as a new lightwell or railings, and local basement policies can change the answer. We check your address and the borough's policy before we draw.
Why are floor lowering and new digs quoted individually?
They depend on the ground, the depth, the neighbours and the structure above, which cannot be known without a visit and often a site investigation. Underpinning and temporary works are designed by an engineer for each site.
Can I use the basement as a bedroom?
Yes, if building control is satisfied on damp, ventilation, light and escape. A bedroom needs a window or door that works as a means of escape, which is often the hardest requirement in a cellar.
Will I need a Party Wall notice for a cellar conversion?
Only if the work comes within the Act: excavation near a neighbour's foundations, or work to a shared wall. An internal conversion that does not touch either may not. We say which applies when we survey.
Does a basement count against permitted development limits?
Householder permitted development covers extensions above ground and certain alterations. It does not give a general right to dig out new underground space, which is why the statute treats it as development.
Basement Conversion Plans: from £2,265 inc survey
(£2,065 if you supply measured drawings)
What you get
- Measured survey
- Drawings
- Structural input
- Borough basement-policy check
- A written view on whether planning permission is needed for your scheme
The total you pay, fixed in writing before you commit. Council fees are separate and paid to the council. Prices checked 24 October 2026.
How our basement conversion plans service works