How much does a basement conversion cost? - Photo

Basement Conversion Cost

How Much Does a Basement Conversion Cost?

Do you have an under-utilised basement space that’s begging to be renovated? This is where Danford, Brewer & Ives, your basement conversion specialist, can make your underground dreams become a reality.

Basements have so much potential. They aren’t just old store cupboards for tumble dryers or ironing boards. With a little vision, planning and help from the experts, your basement can become a fully functioning, practical and liveable addition to your home. And given the lack of land in the United Kingdom, if you can’t build an extension at ground level, then why not look underground?

Your basement could be a whole new living space, but we know you’re wondering: how much does a basement conversion cost in the UK? Keep reading, as we explain everything.

How Much Do Cellar Conversions Cost?

A basement or cellar conversion is one of the best ways to create a new space on your property. But naturally, you’ll want to know roughly how much you might expect to spend on such a project.

You need to remember that a basement conversion is a large building project that requires professional expertise, so it’s important to invest your money wisely rather than simply going for the cheapest option.

The overall cost of basement conversion in the UK will depend on a number of different factors. The biggest factor to consider will be the existence, or non-existence, of available cellar space. If your home already has a cellar space of reasonable size, then it will take less work to shape this into a liveable area. If the cellar space is small, needs extending, or if you need an entirely new space excavated and underpinned, this takes more time, work and money.

The main factors you’ll need to consider when planning the budget for a basement conversion include:

  • The size of the basement conversion
  • The complexity of the basement conversion
  • The type and number of materials needed for construction
  • The interior design, furnishings and decoration needed to complete the basement conversion

You may be quoted a basement conversion cost per square metre, so of course, the larger the project, the more budget that’s needed. For accurate cost estimates, you’ll need to have a basement conversion specialist carry out a detailed survey.

Are Basement Conversions Worth It?

We often see basements as dark, dingy and smelly places that are best avoided, but here at Danford, Brewer & Ives, we want to change your perception of the basement. Basement conversions have so many fantastic benefits, and it’s the potential of this underground space that we love the most.

Here are some of our favourite basement conversion benefits that, in our professional opinion, always make the work and investment worth it:

  • Provides an extra space or living room to your home
  • Allows you to be creative with your new room
  • Save on garden space as you extend below ground rather than horizontally
  • Rent the room out to provide yourself with extra income
  • Increase the value of your property

But what might you do with a basement conversion? A basement conversion can be a whole new living space. Okay, so you’ll probably have to chuck out those old paint tins and toys that are lurking down there at the moment, but once the space has been fixed up, lighting has been added, the basement has been waterproofed and damp proofed, then your options are limitless.

Here are a just a few of the great ideas to inspire your basement conversion:

  • An extra bedroom or guest room
  • A new living room
  • A games, party or entertainment room
  • A new bathroom
  • A spare utility or storage space
  • A work office or writing room

But to realise these benefits, you need someone who can get the job done. And that’s where Danford, Brewer & Ives comes in.

How Long Does a Basement Conversion Take?

A new basement conversion can take anywhere from a few weeks to (more likely) a few months to complete. This takes into account the various stages involved in any basement conversion project, which requires planning, surveying, construction and finally decoration.

There are three primary stages involved in the process. These are:

  1. The initial survey and planning stage. This involves a professional taking measurements, drawing up plans and setting time frames and budgets for the project.
  2. The construction phase. This is when your basement conversion is built and waterproofed.
  3. The final decoration stage. This is when the interior is finished and furnished, and when your basement conversion takes shape.

How long this takes will depend on the extent and complexity of the work required. To understand, it’s important to consider the work that’s required in order to turn a dingy, mouldy cellar into a bright and liveable space.

Much of this is down to waterproofing, sealing, drainage and adding effective lighting. In this respect, there are two main methods employed by a basement conversion specialist in order to seal and waterproof the space. The first is simply sealing the basement using cement-based products that hold water back. This requires excellent drainage outside to work long term, as otherwise the pressure build-up will affect the structure of the property.

A second method is installing a cavity drain membrane. Water ingress is then controlled by a plastic membrane, which is gathered in drains and routed to the external drains using a sump pump. This avoids any build-up of pressure, while an air gap behind the membrane allows the structure to ‘breathe’ and dry out when necessary. This membrane is produced from high quality, high-density plastics such as polyethene, which is long lasting, and vapour and gas-proof. This membrane allows a basement space to be fully sealed, waterproofed and drained, while furthermore, it can be protected against dampness.

Once this stage is complete, the basement can be finished with flooring, electrics and wiring can be installed, lighting can be added and any further plumbing services constructed. After this, the walls can be insulated and plastered, and then the interior design and furnishing can begin in earnest.

As you can see, a lot of work goes into a basement conversion, and each project will have a varied timescale based on the complexity and size of the project.

How Much Will a Contractor Charge to Finish a Basement?

So, how much will a basement conversion specialist charge for the time needed to finish a project? Again, this depends on the complexity of the basement conversion, but with a professional team like Danford, Brewer & Ives, you can only expect to receive the best prices and the best service.

The ‘basement tanking’ work we’ve described above is our expertise, and as a basement conversion is often required as part of a larger project, we’re an ideal partner for builders, architects and property developers to work with.

If you run a company, you can make the most of our expertise and experience by subcontracting basement conversion work to us. If you’re an individual, then we’re also ready to help with personal property development projects.

And as well as providing basement conversion expertise, our team has the capacity to excavate and underpin new basement areas from scratch. Rest assured, we provide value for money as well as efficiency and skill in every project we undertake.

So, how much will a basement conversion cost?

A typical basement conversion in the Yorkshire and Teesside area is usually in a Victorian terrace with an existing space in the basement. These basement rooms are usually cold, damp and badly lit.

Provided the existing head height is adequate the price guide is between £875 – £1,600 per square meter. If the conversion works require any digging out, excavating or underpinning the work becomes more complex and more expensive. Underpinning usually costs between £1,500 – £2,000 per square meter.

To help you understand and get an idea of how much your basement conversion might cost we have provided 4 possible basement scenarios. Each one is based on 30m square of space below ground in a Victorian end terrace house.

Option 1. Simple Basement Conversion

This basement is a single room with 3 external walls and 1 party wall. The head height is more than adequate at 2.2m. The room is currently being used a garden/ storeroom. In the winter periods, water appears to rise up through the stone-flagged floor and is leaving the space continuously cold and damp. There is an existing solid stone staircase leading up to the main house and a small light well to the rear of the room.
Using a cavity drain membrane system, we will transform this space into a warm, dry storage area.

To achieve this we will install a perimeter drainage channel with flushing points at each change in direction. As there is no natural drainage present we will install a sump and pump with a high level battery alarm system. The drainage and pumps are fully maintainable and when combined with cavity drain membranes forms a waterproof structure to comply with current British Standards 8102 (2009) – The code of practice for protection of below ground structures against water from the ground. Your basement is now ready for the ‘fit out’ stage.

A new insulated timber floating floor will be installed and the walls lined with independent timber studs. The 1st fix electrics for the supply of a new socket and lighting circuit will be carried out along with plumbing alterations to add a new radiator. The walls and ceiling will then be plaster boarded and plastered to a finish.

The electric and plumbing works will be finished and signed off as required. The basement conversion works are now complete and ready for decorating. As this space will only be used as storage Building Regulations do not apply and therefore insulation is not a requirement.


  • 4 walls, room area approx. 30m2
  • Cavity drain membranes to walls and floor
  • Drainage channel and mechanical pump system
  • Timber floating floor and studwork
  • Plasterboard and plaster finish
  • Electrics & Plumbing
  • Party Wall agreement
  • 10 year Independent, insurance backed guarantee.

Estimated Cost £ 26,250 + VAT (£875/m2)

Option 2. Standard Basement Conversion

A standard basement conversion would involve the same works as Option 1 with the addition of needing to have Building Regulations as the space is now to be used to provide an extra living space for the property. This will include the addition of insulation within the studwork and ceiling areas. There is also the inclusion of an internal load-bearing wall that would require lining with cavity drain membrane and stud work.


  • 4 external walls (30m2)
  • 1 Internal (load bearing wall)
  • Drainage channel and mechanical pump system
  • Timber floating floor and studwork
  • Insulation
  • Plasterboard and plaster finish
  • Electrics & Plumbing
  • Party Wall agreement
  • Building Regulations
  • 10 year Independent, insurance backed guarantee.

Estimated costs £31,500 + VAT (£1050/m2)

Option 3. Complex Basement Conversion

A complex basement conversion has all the detail as standard basement with the addition of a more sophisticated pump system which includes a secondary pump with battery back-up. This type of basement conversion would also include digging out and creating a new external doorway with concrete access steps. Creating this opening would most likely require planning approval.


  • 4 external walls (30m2)
  • 1 Internal (load bearing wall)
  • Drainage channel and mechanical pump system with battery back up
  • Timber floating floor and studwork
  • Insulation
  • Plasterboard and plaster finish
  • Electrics & Plumbing
  • New external doorway with concrete steps
  • Party Wall agreement
  • Building Regulations
  • Planning permission fees
  • 10 year Independent, insurance backed guarantee.

Estimated cost £48,000 + VAT (£1600/m2)

Option 4. Complex Basement Conversion with Underpinning.

This type of conversion requires increasing the existing headroom. To achieve this the internal floor slab will be excavated to the required depth. The existing foundations will also require extending. This is done using a process called underpinning. With the help of our Structural Engineer we can design and specify the detail required to carry out these works in a safe and controlled manner. Once the underpinning is complete the drainage and membranes are installed and the fit-out completed.


  • 4 external walls (30m2)
  • 1 Internal (load bearing wall)
  • Drainage channel and mechanical pump system with battery back up
  • Concrete floor and studwork
  • Insulation
  • Plasterboard and plaster finish
  • Electrics & Plumbing
  • New external doorway with concrete steps
  • Party Wall agreement
  • Building Regulations
  • Planning permission fees
  • 10 year Independent, insurance backed guarantee.
  • Underpinning and excavation works

Estimated cost £95,000 + VAT

Contact Danford, Brewer & Ives to get the best deal

If you’re planning on building a basement, Danford, Brewer & Ives has an expert team of basement converters ready to assist. Contact Danford, Brewer & Ives today to find out how we can make your basement dreams a reality.

Case Study: Basement Conversion Playroom Harrogate

Basement Conversion Harrogate - Playroom

Danford Brewer & Ives undertook a basement conversion on a property in Harrogate during the spring of 2018.
It is a terrace house, constructed of stone, with a slate roof, brick chimney, UPVC guttering, wooden windows and stone flag floors.
The owner’s intention was to convert the basement into a playroom for his children.  They also wanted a utility room, a toilet and two storage areas/cupboards, each separated by internal walls from a landing that leads to a new staircase.
The process was as follows:

  1. Damp Survey

We first needed to ascertain what level of damp the existing basement had. The survey was restricted to the basement and our readings (taken with the aid of an electrical conductivity-type moisture meter known as a Protimeter) revealed evidence of rising and penetrating damp to all areas, as well as water ingress.
Two major causes of the damp were the high ground levels and structural abutments, so we recommended that either the external levels were reduced or an internal barrier, incorporating a waterproofing agent or membrane, was applied and fixed to those areas - at least up to the level of the newly installed damp-proof course.
(NB We did not undertake a structural survey and always recommend that any worries/queries regarding such issues should be resolved using a suitably qualified person.)

  1. Structural and basement waterproofing

All structural and basement waterproofing work was undertaken in accordance with British Standards Document BS 8102:2009, the code of practice for the protection of below-ground structures against water.
It specifies a level of protection (grading from 1-3) based upon the end use of the conversion. In this instance, since it is being used as habitable accommodation, the required grade was 3.
One assumption BS 8102:2009 makes is that ingress will occur during the conversion’s lifetime - caused, for example, by a change in the water table, drains becoming blocked or heavy rainfall causing localised saturation.
After careful consideration of various methods of controlling persistent water entry, therefore, we recommended installing our cavity drain system to all areas indicated on the plan.

  1. Drainage

The cavity drain system’s effectiveness is based upon discharging free water, before pressure builds behind the system.
For this to be possible, an effective and fully maintainable drainage system was installed.

  1. Preparatory Work

The existing ceilings were stripped, with plumbing and electrics relocated in the ceiling.
Some timber repairs were required in the utility area. Where there were signs of deterioration, the timbers were cut back and replaced by either steel support brackets or resin ties.
The existing opening into the floor void was increased to approx. 2.4m wide by installing a new concrete lintel (that was subject to a structural engineer’s approval). An area, approx 1200mm back into the void, was then lined with a membrane and framed out to form a new storage area.
The existing single skin brick wall in the utility area was removed (no structural replacement was required) and the stone staircase was broken out and also removed.
We lifted the existing stone flagged flooring, which was kept for the client, and the floor was excavated by a depth of approx. 100mm to allow for the build up of the membrane and floor finishes.

  1. Waterproofing

A sacrificial screed was poured incorporating a 100mm x 75mm perimeter rebate. Once the floor was cured and prior to membranes being installed, the concrete was treated with an anti-lime coating to stop the build-up of free lime within the cement and prevent the drainage from blocking.
The walls were lined with an 8mm studded Cavity Drain Membrane. This was installed to at least the height of damp course level.
A perimeter drainage channel was installed, complete with servicing flushing points. The drainage falls and collects within the sump chamber and any water build-up is discharged via the twin pumps located within. Each pump has its own independent fused spur supply.
Once the drainage and pumps were installed, we flood-tested the system.
The floors then received a 20mm CDM which was sealed to the wall membrane as required. The system was then flood-tested again.

  1. Fit out

To be compliant with current building regulations, and as the basement is a habitable space, insulation was required (our specification was subject to Building Control approval).
We laid a minimum 50mm close cell insulation board on top of the floor membrane and covered it with a 22mm water-resistant floorboard.
The external walls of the basement were lined with 75mm timber frames and fitted with 75mm rigid foam boards. The internal walls were battened out using 50mm x 25mm timbers.
We also installed a new timber staircase.
A toilet area was formed under the stairs, supplying a new WC and basin. The foul waste collects in a chamber within the utility room and discharges into the nearest foul waste drains externally.
The ceiling received a 100mm wool insulation and all surfaces were then lined with a 12.5mm plasterboard.
The rooms were fitted out with a softwood skirting board. New doors were fitted to both the living area and utility room.

  1. Fixing to or through the waterproof systems

When fixing through the membranes, careful consideration was taken, including the possibility of an alternative method.

  1. Non-earth retaining walls

We carefully marked the position of the fixing at the required point and drilled. We filled the hole with a high-quality mastic and then inserted a propitiatory plastic fixing plug. More sealant was applied around the hole to form a seal before inserting the screw.

  1. Earth retaining walls

Specialist fixing plugs with purpose-made seals were used in order to maintain the integrity of the membrane.
It is important that the flushing/service points are easily accessible so that regular maintenance can be carried out to the drainage system to ensure that no blockages occur. Maintenance programs are available and are a condition of the guarantee.
As the works described involve a Party Wall, the Party Wall Act 1996 was therefore applied. This requires the owner of a property to notify his/her neighbour(s) of the proposed works and obtain the neighbours’ consent. A neighbour cannot unreasonably withhold consent, but should you require further advice or information, please initially contact our office or a suitably qualified party wall surveyor.
Danford Brewer & Ives are very aware of Health & Safety issues and dangers within our industry. Any work that we carry out is done so safely, in the best interests of our customers, the general public, and ourselves.
Upon completion of our specialist remedial works and full settlement of our final invoice, Danford Brewer & Ives issue a 10-year independent insurance-backed guarantee.
The client was thrilled with the final results and tells us the children are using their new playroom all the time.  If you have any plans to convert your basement into additional living space then contact Danford Brewer & Ives for expert advice and support.
Here is a short video showing how the project progressed and finished up.