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What changes the cost of a cellar conversion?
A cellar conversion quote depends mainly on how wet the cellar is, whether the floor must be lowered for headroom, how light, air and a fire escape route will be provided, and what approvals are needed. This guide prints no price per square metre, so compare what each quote covers.
Last reviewed:What is a cellar conversion?
A cellar conversion turns the space under a house into a room you can live in: a snug, an office, a utility room, a gym or a bedroom. It differs from building a new basement, because the space already exists. The work is about making it dry, safe, warm and usable.
Some cellars need little more than lining and wiring. Others need the floor dug down, the walls supported, new windows cut, and a full waterproofing system. That is why there is no single price for this job.
This guide does not print a price per square metre or a typical total. It explains the parts that move a quote, so you can compare builders and specialists on the same terms.
QuoteZila starts in Cheshire, with Crewe, Nantwich and Sandbach first, and anyone in the UK can post a cellar job.
Do you need planning permission or building regulations?
Planning. The Planning Portal says you are unlikely to need planning permission to convert an existing basement into living space, and that planning permission will not be needed if you are just redecorating an existing space [S1]. It says planning permission will be needed where you create a new unit of accommodation, change the use significantly, for example to run a business, add a lightwell, or alter the external appearance of the property [S1]. If the house is listed, or in a conservation area, check with your local planning authority before anything else.
Building regulations. The Planning Portal says building regulations apply to works on a basement, and cover areas such as fire escape routes, ventilation, ceiling height, damp proofing, electrical wiring and water supplies [S2]. It adds that underpinning and foundation work may also be needed [S2].
GOV.UK says building regulations approval is different from planning permission, that you might need both, and that you should check with a building control body if you can't decide [S4]. GOV.UK: building regulations approval. QuoteZila is not affiliated with this organisation. GOV.UK explains when you need building regulations approval.
Party walls. The Planning Portal says to consider the Party Wall Act 1996 if other properties adjoin yours and you share walls [S2]. GOV.UK says you must tell your neighbours if you want to carry out building work near or on a party wall in England and Wales, and that this is separate from planning permission and building regulations approval [S3]. In a terrace or semi, this can add time and surveyor fees to the job.
What are the big cost drivers?
How wet is the cellar?
This is usually the largest single factor. Many cellars are damp because the walls and floor are below ground and in contact with wet soil. Turning them into a room means keeping water out, or managing it so it can't reach the room.
Two broad approaches are common:
- Barrier systems, sometimes called tanking, where a waterproof coating or render is applied to the walls and floor.
- Cavity drain membranes, where a dimpled membrane lines the walls and floor, and any water behind it is led to a drainage channel and, often, a sump and pump.
Which one suits your cellar is a design question for a specialist. Ask each quote to name the system, explain why it was chosen, and say what maintenance it needs. A pump needs power, servicing, and a plan for when the power is off.
Is there enough headroom?
The Planning Portal lists ceiling height among the things building regulations cover for basements [S2]. If the cellar is too low, the options are to lower the floor or accept a storage space rather than a habitable room.
Lowering a floor means digging out, taking soil away, and often supporting the walls so the house above isn't undermined. That may mean underpinning, which is specialist structural work. What changes the cost of underpinning covers it.
Light, air and a way out
A room needs daylight, fresh air and, depending on its use, a safe way out in a fire. The Planning Portal lists fire escape routes and ventilation among the areas the regulations cover for basements [S2]. In practice that can mean enlarging an existing window, adding a lightwell, fitting mechanical ventilation, or changing the stair. Adding a lightwell is one of the things the Planning Portal says needs planning permission [S1].
The walls and floor themselves
The Planning Portal says that in conversion projects, existing walls need to be checked for weight and structural stability, weather resistance including damp-proofing, and thermal resistance [S9]. Old cellar walls may need repair, a new inner lining, insulation and a new floor build-up before they are ready for plaster and paint.
Services
Most cellars need new lighting and sockets at least. Many also need heating, and some need water and drainage for a utility room or shower room. A drain below the level of the main sewer may need a pump. Electrical work should be done by a registered electrician who can certify it.
What should a cellar conversion quote list?
| Line | What a clear quote says | Why it changes the cost |
|---|---|---|
| Survey | Who assessed the damp and the structure, and what they found | The design rests on it |
| Waterproofing | Named system, walls and floor, sump and pump if any | Usually the largest single item |
| Digging out | Whether the floor is lowered, by how much, and soil removal | Labour, skips and structural support |
| Structure | Underpinning, new beams, opening up walls or windows | Needs engineering design |
| Light and escape | Windows, lightwells, stair changes | Planning and building control both care |
| Ventilation | Natural, mechanical, or both | Keeps a below-ground room dry and fresh |
| Insulation and lining | Wall and floor build-up | Comfort and heat loss |
| Electrics | Lighting, sockets, certificate | Registered electrician needed |
| Heating and plumbing | Radiators, water, drainage, pumped waste | Each adds a trade |
| Finishes | Plaster, flooring, decorating, doors | Often priced separately |
| Approvals | Building control, planning, party wall | Fees and time |
| Waste | Soil and rubble removal | Disposal is the trader's waste [S8] |
| VAT | Rate and whether it is included | Standard rate is 20% for most work on houses, with some exceptions [S5] |
How does VAT affect the total?
GOV.UK says VAT for most work on houses and flats by builders and similar trades is charged at the standard rate of 20%, but there are some exceptions [S5]. It says a reduced rate of 5% may be charged for some types of work if certain conditions are met [S5]. Whether any reduced rate applies to your cellar depends on the details, so ask each builder which rate they will charge and why.
Smaller firms may not be VAT registered at all. GOV.UK says a business must register for VAT if its taxable turnover for the last 12 months goes over £90,000 [S6]. Check each quote says whether VAT is included, so you compare like with like.
Who should you ask to quote?
A builder with cellar or basement experience, who can manage the structure, the trades and building control. Our builders page covers the trade.
A waterproofing or damp specialist, sometimes alongside the builder, for the waterproofing design. Our damp proofing page covers that trade, and rising damp treatment explains a different, above-ground damp problem.
A structural engineer, if the floor is to be lowered, walls opened up or the house supported. What changes the cost of a structural engineer covers that.
An electrician, a plumber or heating engineer, and a plasterer for the later stages. Our electricians page covers that trade.
How do you compare cellar conversion quotes?
Give each builder the same details: the cellar's size, what you want to use it for, how high the ceiling is, whether it ever takes in water, how you get in, and photos of the walls, floor and any existing windows or grates.
Citizens Advice says a quote is a fixed price, while an estimate is just a rough guess, so you might end up paying more [S7]. It suggests getting written quotes from at least 3 different traders, and being cautious if one price is a lot lower than the others, because it might not be for exactly the same work [S7]. It says that if a deposit can't be avoided, not to agree to more than 25% [S7].
Also ask:
- who designs the waterproofing, and whether there is a written warranty for it
- whether the structural design comes from an engineer, and who pays for it
- who applies to building control, and when inspections happen
- how long the job will take, and whether the house stays usable during it
Jobs you might post
These are examples, not taken from real job posts.
- "Victorian terrace with a cellar under the front room. Dry most of the year but damp in winter. Want it made into a home office."
- "Cellar has low headroom. Please quote for lowering the floor and making it into a usable room, including engineer and building control."
- "Cellar already lined years ago but smells musty. Want someone to look at ventilation and the waterproofing."
Posting a cellar conversion job
Photos of the cellar walls and floor, the ceiling height, and a note of when it gets wet are the most useful things to add. Your job goes to up to five matched local businesses [QZ2]. QuoteZila is free for customers and local businesses [QZ1].
Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.
Posting a job is not open yet
Questions about cellar conversions
How much does a cellar conversion cost?
This guide does not print a figure. A dry cellar with good headroom that only needs lining, wiring and decorating is a very different job from a damp cellar where the floor has to be dug down and the walls underpinned. Ask each builder to price the waterproofing, structural work, services and finishes separately.
Do I need planning permission to convert a cellar?
Often not. The Planning Portal says you are unlikely to need planning permission to convert an existing basement into living space. It says permission will be needed if you create a separate home, change the use significantly, add a lightwell or alter the outside of the property.
Does a cellar conversion need building regulations approval?
Yes. The Planning Portal says building regulations apply to work on a basement, covering areas such as fire escape routes, ventilation, ceiling height, damp proofing, electrical wiring and water supplies.
What if my cellar floods sometimes?
Tell every builder before they quote. A cellar that takes in water needs a waterproofing design that deals with it, and may need a pump. A quote that does not mention how water will be managed is incomplete.
Do I need a party wall agreement?
If you share a wall with a neighbour, possibly. The Planning Portal says to consider the Party Wall Act 1996 if other properties adjoin yours and you share walls. GOV.UK says you must tell your neighbours about building work near or on a party wall in England and Wales.
What VAT applies to a cellar conversion?
GOV.UK says VAT on most building work on houses and flats is charged at the standard rate of 20%, with some exceptions. It says a reduced rate of 5% can apply to some types of work that meet certain conditions. Ask each builder what rate they will charge and why.
Does QuoteZila check builders?
No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.
This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.
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Sources
- QZ1: QuoteZila product fact. QuoteZila home and FAQ copy.
- QZ2: QuoteZila product fact. QuoteZila FAQ How does getting quotes work.
- S1: Basements: planning permission, Planning Portal. Accessed 2026-11-02. https://www.planningportal.co.uk/permission/common-projects/basements/planning-permission/
- S2: Basements: building regulations, Planning Portal. Accessed 2026-11-02. https://www.planningportal.co.uk/permission/common-projects/basements/building-regulations/
- S3: Party walls and building work, GOV.UK. Accessed 2026-11-02. https://www.gov.uk/party-walls-building-works
- S4: Building regulations approval, GOV.UK. Accessed 2026-11-02. https://www.gov.uk/building-regulations-approval
- S5: VAT for builders, GOV.UK. Accessed 2026-11-02. https://www.gov.uk/vat-builders
- S6: Register for VAT, GOV.UK. Accessed 2026-11-02. https://www.gov.uk/register-for-vat
- S7: Before you get building work done, Citizens Advice. Accessed 2026-11-02. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/
- S8: Waste duty of care: code of practice, GOV.UK (Defra). Accessed 2026-11-02. https://www.gov.uk/government/publications/waste-duty-of-care-code-of-practice/waste-duty-of-care-code-of-practice
- S9: Is building regulations approval needed for work to external walls?, Planning Portal. Accessed 2026-11-02. https://www.planningportal.co.uk/permission/common-projects/external-walls/building-regulations/