With increasing house prices, it’s not always possible to move house when you want extra room in your home. Even building an extension might not be feasible if your property’s curtilage isn’t large enough. But, there’s another option which you might not have thought about; converting your cellar or basement.
Typically, the average cost of cellar conversions (existing) in the UK ranges from £900-£1,800/m2 to produce a shell ready for fitting-out. Therefore, an average cellar shell of 5mx4m costs £18,000-£36,000. However, to excavate the floor, thus increasing headroom, you must underpin foundations costing £1,200-£2,500/m2 or £24,000-£50,000 in total.
How Much Do Cellar Conversions Cost?*
The cellar conversion prices shown in the table below allow you to compare your basement area. So, use them to calculate an approximate cost of a cellar conversion. Also, the estimated cellar renovation cost assumes a rate of £1,500/m2.
|Size of Cellar||Estimated Cost|
|3m x 3m (9m2)||£13,500|
|3m x 5m (15m2)||£22,500|
|4m x 4m (16m2)||£24,000|
|4m x 5m (20m2)||£30,000|
|4m x 6m (24m2)||£36,000|
|5m x 5m (25m2)||£37,500|
|5m x 6m (30m2)||£45,000|
|5m x 7m (35m2)||£52,500|
*We compiled these figures from various sources. Every property and basement is unique, so use them as estimated costs when searching for a reliable contractor. Furthermore, the estimated price disregards underpinning and fitting-out like constructing stud partition walls and installing lighting and central heating.
Basement Conversion Cost Factors
Several factors determine the cost of a basement conversion. Usually, these vary depending on your property.
Sometimes, you might have an existing basement that is already waterproof and fitted out with electricity. If so, it’s a relatively simple task to build stud partition walls. Moreover, you can easily install up-market light fittings, decorate, and carpet the room. And sometimes, you might even have windows already installed. However, it’s not often you get this lucky.
Usually, the cellar is dark, damp and uninhabitable, and you must do a lot of work to make it a pleasant place.
For example, suppose your project is a 16m2 terraced house cellar conversion. In this case, you pay about £24,000 for the basic conversion, plus a Party Wall Agreement (£700+). You will need a structural engineer (£1,000-£1,500) and surveyor (£500-£1,600), possible planning permission (£200-£290), definitely building control approval (£750+), architect (5-7% of project cost), and all the extras like carpentry, electrics, plumbing, and flooring.
Labour costs in London and Southeast England are up to 20% more than elsewhere. So, the 16m2 conversion will cost almost £30,000 to create a waterproof shell.
The size of the basement will have a significant impact on the overall project cost. Usually, you need more labour and materials to convert a large cellar than a small one.
Age of property
If the building is more than 50 years old, expect more work to make the room habitable. For example, a Victorian cellar conversion will probably suffer from crumbling brick, no damp proof course, no electricity and a beaten earth floor. In comparison, a basement dating to the 1970s will already have good foundations, a concrete floor and damp proofing, electric light and possibly internal plumbing.
Fixtures and fittings quality
Sometimes, you can reduce the project cost by choosing poor quality fixtures and fittings. But, over time, you’ll find they wear out quicker and need constant maintenance and replacement. Usually, inferior quality materials work out more expensive over the lifetime of the conversion.
Fitting windows & external doors
Natural light is always a benefit in a room, and external windows or doors provide an evacuation route in case of fire. Therefore, this helps with Building Regulations compliance. But, digging out light-wells, fitting windows and providing separate access and steps to ground level will increase the cost significantly. Typically, to supply external doors costs £5,000-£10,000 per exit, while egress windows cost £1,200 to £4,000.
Traditionally, cellar ceilings weren’t built to a standard height. However, for comfort, maintain a distance of approximately 2.4m from the finished floor to ceiling.
Digging out a cellar floor needs somewhere to put the soil. Moreover, it also requires an underpinning of the building’s foundations. Typically, this costs an additional £500-£700/m2. However, increasing the headroom in a cellar might need planning permission, as the local authority may regard the conversion as an extension. Therefore, always check with your local planning department before starting.
While we’re discussing possible planning permission, you usually won’t need it unless you increase the headroom. Local authority prices for a planning application differ across the various nations of the UK. Typically, Scotland is the cheapest at £202. Next, comes England with £206. Then, Wales at £230. Finally, Northern Ireland costs £290.
We’ll talk more about planning permission later.
Party Wall Agreement
As we mentioned earlier, if you have a terraced or semi-detached house and want to convert its basement, you must enter into a Party Wall Agreement before beginning work. Specifically, you need an agreement if your project includes:
- Excavating within 6m of your neighbour’s property at an angle of 45 degrees from the base of their foundations.
- Excavating within 3m of your neighbour’s property at a level lower than their foundations.
- Inserting holes in the shared wall to insert joists, lintels, flashing or anything else.
The Party Wall agreement costs around £750. Also, you need two surveyors with fees of £150-£250/hr for up to 20 hours, or £3000-£5,000 each. However, if you’ve got an accomodating neighbour, you might only pay £500-£1,500.
Structural engineer & architect
Structural engineers cost £100-£200/hr or £1,000-£1,500 for typical calculations involving load-bearing walls and RSJs.
In comparison, architects generally charge 5%-7% of the project cost. So, for a project costing £30,000, the architect charges between £1,500-£2,100. Alternatively, if yours is a small project, they will charge a flat fee.
By services, we mean electricity, drainage, plumbing, and central heating. And this depends on the proposed purpose of the conversion. Usually, central heating and electricity should be enough if the room is a living room or family den. In comparison, a kitchen or a bedroom with an en-suite toilet need drainage, plumbing, central heating and electricity.
If your basement is below the local water table, expect it to be, at best, very damp. Sometimes, when waterproofing the room, you might need to install water pumps and drainage to keep the water level at bay and the space habitable.
Expect to create a lot of waste when converting a basement, especially if you dig to create more headroom or add external doors. Usually, a building contractor will include this within their quotation. However, if it’s separate, expect to pay from £100/week for a mini-skip up to £400/week for a large skip.
Fitting out the room
The overall cost of a basement conversion wouldn’t be complete without fitting-out the room. Generally, this depends on its intended purpose.
- Flooring – Every room needs a floor. Basic laminate or carpet sells for £10-£20/m2. Alternatively, more up-market flooring costs £40-£70/m2.
- Central heating – a couple of new radiators added to your current central heating system costs about £500.
- Bathroom – A standard bathroom costs £5,000-£10,000.
- Kitchen – A kitchen costs £5,000-£25,000, depending on quality and size.
- Electrician – An electrician charges £150-£200/day for everything electrical, plus materials as extra.
- Ventilation – Keeping fresh air circulating and removing moisture is essential in a basement. You can use opening windows, extractor fans, and air bricks to provide ventilation. The choice depends on the design, your budget, and the amount of moisture present.
UK Planning Permission & Building Regulations
A cellar or basement isn’t habitable unless it conforms to the current Building Regulations and other legislation. Therefore, comply with the following legislation.
Suppose you live in a semi-detached or terraced property. Therefore, your home shares particular walls and foundations with your neighbour. So, if you intend to alter these in any way, you must conform to the Party Wall etc. Act 1996. As usual, The UK government publishes guidance to help you comply with the legislation.
Generally, you won’t need planning permission for a cellar conversion unless:
- It is a separate building.
- The usage is significantly different. For example, if you intend to change its use from residential to industrial or commercial premises.
- The structure is a listed building.
- You intend to change the outside appearance. Perhaps, by adding windows or a light-well.
All work necessary to convert a cellar or basement into a habitable area must comply with the UK Building Regulations. If in any doubt, contact your local authority and speak to Building Control.
Generally, basement conversion must comply with the following:
- Part A. The basement’s foundations, load-bearing and party walls must be structurally sound.
- Part B. All materials must have 30-minutes fire resistance. Furthermore, there must be a suitable fire escape.
- Part C. The conversion must be proof against water and other contaminants in the soil.
- Part D. Use approved non-toxic waterproofing materials according to the manufacturer’s instructions
- Part E. Bedrooms and toilets must be insulated against sound transmission. Moreover, no more than 45dB of sound can escape the area.
- Part F. The spaces must have ventilation to provide fresh air and remove excess moisture.
- Part G. Sanitation, hot water safety and water efficiency. You must install all hot and cold plumbing and drainage to comply with the regulations.
- Part H. All drainage must comply with the regulations.
- Part J. If there is a water or heating appliance in the basement, its installation and fuel supply must comply with this regulation.
- Part K. Protects from falling, collision and impact. Therefore, all glass windows must be Fensa approved. Also, stairs must comply with the relevant dimensions.
- Part L. You must thermally insulate the basement conversion to the required standard.
- Part M. Access and egress must comply with the regulations.
- Part P. All electrical work is to be installed by a registered electrician.
- Part Q. External doors and windows must have working locks.
- Regulation 7. Materials and workmanship to be of good quality.
The following stages highlight how long it takes to convert a basement into a habitable room. Remember, these estimated durations depend on the size of the cellar and the complexity of the project.
- First, Stage 1. – Assuming you need planning permission, the planning, design, and local authority approval process takes about 4-6 months.
- Then, Stage 2. – Structural excavation, underpinning, and waterproofing take 2-5 months.
- Finally, Stage 3. – Fitting-out the basement shell into a usable and comfortable living area takes 2-4 months.
How Do You Convert a Cellar?
All cellar and basement conversions are unique, so we only consider conversion in general steps. Perhaps, not all these are necessary.
- Check with the local authority about the planning and building control regulations.
- Hire specialists to determine soil type and drainage.
- Conduct a tree survey and decide whether the conversion will affect them.
- Use a surveyor to negotiate a Party Wall Agreement with the neighbours.
- Hire an architect to draw plans and a structural engineer to calculate structural and foundation loads.
- Determine the conversion’s area.
- Determine the required headroom.
- What is the basement’s purpose?
- Determine the staircase position.
- Is natural light necessary?
If the conversion needs headroom excavation or to pour a new concrete floor, you will probably have to install new foundations or underpin the existing ones. This is a specialist job. Don’t try it yourself.
Build floors and walls
- Lay drainage pipes as necessary.
- Lay hardcore and concrete floor.
- Build new walls and prepare existing walls.
- Install windows and external doors if necessary.
- Insulate and waterproof flooring and walls.
- Install staircase.
- Plasterboard and plaster walls and ceiling.
- Construct stud partition walls.
- Install plumbing, electricity and central heating.
- Paint & decorate.
- Add floor covering.
Cellar Conversion FAQ
How much value does a cellar conversion add?
If there are no problems, a cellar conversion can increase your property’s value by between 15% and 30%.
Is it worth converting a cellar?
Definitely, the average cost to convert your cellar or basement into habitable rooms will be much less than the increase in value of your home. However, to conform to the Building Regulations, you must use materials conforming to the British Safety Standards, and use competent basement conversion specialists or qualified general builders. So don’t be tempted to build it on-the-cheap.
Can a basement be a bedroom (UK)?
Yes. However, specific stipulations apply for all rooms in a basement.
All basement conversions require a Building Notice submission and comply with the Building Regulations if the conversion provides or contains one of the following: a fixed staircase, office or study, playroom or hobby room, en-suite bathroom, or a bedroom.
Asking a reputable cellar conversion specialist or general builder for cellar conversion quotes can be a problem if you don’t know what you’re doing. This job is not something for the average DIY hobbyist, and you should always use a qualified and registered professional. Also, be wary of companies who reckon they can convert your cellar into a habitable room for less than £3,000. They can’t, no matter how small the basement is! There are just too many Building Regulations to comply with.
Instead, let us find a competent basement conversion specialist or experienced general builder. Complete the form on this page, and we’ll ensure you receive up to four quotes from companies that know what they’re doing.