Key Factors in your Basement Conversion Cost

A conversion of an existing cellar into a liveable space is a much simpler job than excavating a new basement, and so it’s essential to understand what the costs are.

Basement conversions generally require a schedule of works including:

  • Excavations if a new cellar is required or the existing space needs to be expanded.
  • Adding structural supports and underpinning to stabilise the property.
  • Carrying out waterproofing to protect against water and dampness.
  • Installing plumbing pipes, gas and electrical wiring.
  • Building the flooring and plastering walls ready for decorating.
  • Final decorating and installing heating appliances and light fixtures.

Costs will depend very much on your basement’s size, the extent of the excavations, and what finishes you require in your new cellar conversion.

Excavations can be expensive, costing around £1,920 to £2,640 per square metre, including the excavation and underpinning. This cost will be avoided entirely for a property with an existing cellar, so it will be the main expense for a home without a basement space.

Your property might need structural supports for an existing basement that hasn’t been in use or needs additional underpinning to support the property’s weight. Poor condition basements may also require extra work to ensure they are watertight and liveable, with critical jobs providing the space with light, ventilation and protection from dampness.

The size of the basement is also important, whether for a conversion of an existing space or constructing a new one. That dictates how much renovation is required, the number of days of labour, and the cost of the materials.

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Other Budget Considerations in Converting a Basement

UK average basement conversion prices range from £1,000 to £1,800 per square metre for the conversion work (excluding excavations), but so much depends on the age of the property.

Many Victorian homes have cellars used for storing coal or wood or with a heating space or open fire. If the basement is structurally sound, the costs will be much lower than for a neglected area, which needs extensive work to ensure it is safe and habitable.

Other charges all rely on your plans for your new cellar conversion – for example:

  • Fitting plumbing and pipework will be essential to ensure the space is warm, and you might need to add the Cost to Move a Radiator to the budget.
  • If you have an existing Hive Installation, you will need to expand this to cover the basement’s central heating appliances or consider installing a new system to manage the property heating from one control point.
  • Larger basements may require a significant amount of heat, particularly when they are first used since the walls and ceiling will not have retained any thermal warmth. The area can feel very cold and damp unless it is adequately heated. It is usually advisable to book a Boiler Inspection to make sure your heating has the capacity to cope with another level and additional radiators.
  • Fitting a kitchen or full bathroom will also add to the price, with the costs of the fixtures and materials, hire charges for a professional plumber or electrician, and decorative work such as tiling.
  • Glazing in the roof or lightwells usually needs reinforced glass to ensure it is strong enough to cope with the weight of people walking over it, and so the more extensive the areas of glazing, the higher the overall cost.

It is well worth requesting a survey before looking at basement conversion costs since you will need a professional to establish whether:

  • The property has solid concrete or timber subfloors underneath your ground floor.
  • Any specialist access equipment is required, particularly in urban areas or for terraced properties without rear access.
  • Ground conditions, with complex scenarios including clay, sand or marshland.
  • There are drains underneath the property, which will need to be diverted to avoid damage during the building work.

Finally, the contractor will tailor your access requirements to the type of property you own. That might mean opening up your garden and re-levelling to install Bifold sliding doors – in which case landscaping work and a retaining wall will likely be needed.

A stairwell with external access is ideal if the basement is to be used as standalone living accommodation. However, you will need to budget for a security door and locks, given that the stairwell will allow immediate access into your home.

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Breaking Down the Basement Conversion Cost

We’ve explored how diverse the range of basement conversion projects can be, so it’s essential to have a firm idea about the scope of the work involved and obtain at least three quotes to be confident you are paying a fair price.

More complex home improvement works such as a basement conversion will usually come with a full breakdown of the costs and subcontractor services involved. It’s best to discuss this with your builder to make sure you know about any additional works you might need that haven’t been included.

A typical basement conversion quotation might look like this:

  • Converting an existing cellar – while the average cost is £1,400 per square metre, you’d expect the quote to be calculated based on the exact dimensions of your basement.
  • Lowering floor levels/underpinning – this costs around £1,500 to £2,000 per square metre, but again your quote should reflect the actual size of the project.
  • Digging new basement spaces – usually between £1,500 and £3,000 per square metre, including labour and equipment hire. Make sure your quote details whether the removal of the waste soil is included!
  • Building a new external stairwell or private access point will be bespoke to the property but would be around £5,000 to £7,500. Installing a lightwell would come at a similar cost.
  • Engineer’s fees – you will need a structural engineer to plan the works and ensure all the right materials are used. This service costs around £1,500 to £2,000 and can be calculated based on a percentage of the value of the work. If you are quoted an hourly rate or a percentage, it’s best to ask for this to be calculated so you have an exact cost.
  • VAT – chargeable at 20% by any registered tradespeople; please check whether the quote includes or excludes VAT.
  • Building regulations approval – some contractors can certify their own compliance with building regulations. In other cases, an inspector will need to visit the property and confirm whether the work reaches the standards. This process can cost from £750 and upwards.

Other costs such as planning permission applications, and a party wall agreement (if required) won’t generally be involved in the contractor price.

Party wall agreements can cost around £1,000 per neighbouring property from whom you require permission, and planning permission is usually about £200.

If you need more living space for an expanding family, it can also be much cheaper to convert a basement rather than moving to a larger home or building an extension outwards, which would eat into the garden.

FAQs - Converting a Basement

Let’s run through some of the most commonly asked questions about converting a basement!

Do I Need Planning Permission to Convert a Basement?

Possibly. If you have a basement space and want to convert it into a new use, you probably won’t need planning permission.

For example, constructing a games room, workshop or home office won’t need a change of use approval for a cellar already in a residential property.

However, if you wish to excavate a new cellar, you will need approval, building regulations certification and assistance from professionals such as architects and structural engineers.

Using a cellar to run a business may require approval for a change of use from a residential space to a commercial one.

How Long Does It Take to Convert a Cellar?

Projects can vary significantly in length – converting one simple cellar room might take a week or two, whereas lowering floors, excavating and installing new supports could take four months or so.

It is hard to give an accurate estimate because cellar conversion projects will all be different. Suppose you have an existing cellar room that requires damp treatments, plastering, decorating and new utility wiring. In that case, the project will take a fraction of the time needed to build a complete new cellar space under a property without one.

Can I Add a Cellar to a Property Without One?

Sometimes, yes. Most cellar conversions are carried out where the property has storage space or a disused basement underneath that needs converting into a liveable space. However, you can sometimes add a basement to a property without one.

This project involves digging out the soil underneath the house and installing supports and foundations to ensure the home’s structural integrity is protected. Excavations can be costly and expensive, and you will need assessments to check whether the property is suitable.

Homes in areas with flood risks may not be suitable, and land drains are vital for properties sitting on heavy clay without appropriate natural drainage.

Are Cellar Conversions Prone to Damp?

They can be, yes – because the cellar conversion is underground, it can be at an increased risk of damp. You can also expect moisture problems in basements that haven’t been in regular use.

The best way to avoid damp problems is to ensure the cellar conversion is carried out by a professional contractor with waterproof treatments and sealant to protect against water ingress.

How Much Does It Cost to Convert a Basement?

The average cost of converting a basement is around £1,400 per square metre – so it depends on the size of the cellar, how much excavation work is required, whether there is a pre-existing basement space, and what level of waterproof treatments and utilities are required.


Rich Crossley
Having spent 30 years working all over the world for top-tier investment banks, I’ve owned and developed houses all over the world – Europe, the US and Asia. I’m now based back in the UK and involved in the property industry – oh, and I’m a keen DIY enthusiast!
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