If you can’t find the space above ground, extending your house into the space below it might be the best option for adding value and space to your property. If you have chosen to convert your basement, there are a lot of questions that you will need to answer before you start.

It is much easier to add a basement if you are also adding a new extension at the same time, while retrofitting a basement might be more costly and more difficult to achieve.

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How much will a basement conversion cost?

Adding a basement under a new extension

Constructing a basement from scratch underneath a new extension will cost £750-1,300 per m² plus VAT. This is for a shell space only – leaving concrete walls and floor ready for the first fix, i.e. when wiring and plumbing goes in ahead of plastering and other surface finishing.

Factors that will influence these costs include: digging below the water table; difficult ground conditions, e.g. if it’s unstable or is solid rock; a need to divert existing services, such as drains; poor access; a long distance to the nearest tip; and the need to underpin parts of the existing building.

Adding a basement under an existing structure

Constructing a new basement under an existing house and underpinning the walls will cost £1,600-2,200 per m² plus VAT for the shell space. As well as the factors above, affecting these costs are the need for any party wall agreements with neighbours and a ground-borne concrete ground-floor structure, rather than a suspended timber-joisted floor.

Turning an existing cellar into a living space

Turning an existing cellar into habitable space by tanking and insulating the walls will cost £450-650 per m² plus VAT, again to first-fix stage. The same factors will increase the costs, including the need to move services, e.g. relocating electricity or gas meters, and also if you’re adding lightwells and a new entry, and lowering the floor level to increase ceiling height.

All these cost guides exclude design and engineer’s fees. If you use a tanking or basement specialist, they may include engineer’s drawings and calculations as part of a design-and-build contract. They may also include an insurance-backed guarantee scheme.

Do I need planning permission to convert my basement?

Converting an existing residential cellar or basement into a living space is, in most cases, unlikely to require planning permission. This is provided it’s not a separate unit and that the usage and external appearance of the property are not significantly altered.

Major works to excavate a new basement, adding a new separate unit of accommodation and/or altering the external appearance of your house, such as adding a lightwell, is likely to require planning permission. As always, Building Regulations will apply, and owners need consent for any internal or external work on listed buildings.

In all circumstances, you should contact your local planning authority for guidance before starting any work. You can apply for planning consent yourself, or employ a professional architect or specialist basement company. Because basements often have little visual impact on a house and don’t overlook neighbours or block their natural light, they are considered to be non-controversial forms of development.

Basements in conservation areas

If you live in a designated conservation area, you will need to find out whether there is any general objection to basements and, if not, what guidelines have been laid down relating to external appearances.

If your property is a shared building or has shared grounds, or you are a leaseholder, you should serve the appropriate notices on all the other parties as part of your planning application.

As a leaseholder, you will also need to fill out a form to confirm you have given notice to the freeholder. This really means confirming you have written to them alerting them to your plans. The freeholder’s consent in principle must be obtained before applying for planning permission. Most leases do not allow structural alterations to be made without this, so refusal would put a halt to the whole project.

Building regulations

The Party Wall Act provides a legal framework for preventing and resolving disputes in relation to boundary walls, party walls and excavations near neighbouring buildings. Unless you live in a detached house, you will need to reach an agreement with your neighbours over shared walls and the correct notices must be issued and consents obtained from them at least two months before work begins.

To comply with Building Regulations and to ensure cellars and basements are inhabitable, your walls may have to be underpinned and the floor dug out further in order to achieve sufficient head height in the new space. Because of this, you may need to add structural supports, such as steel beams.

Any structural work you carry out to your basement room will need to be passed by the local authority building inspector, to make sure that it won’t impact on the rest of your house or your neighbours’ properties.

A basement normally only has one access point in and out of the room. If you are planning to use the space as additional living accommodation, however, you will need to provide an additional means of escape in case of fire. An emergency escape route can take the form of a window or another door, as long as you can demonstrate that, in the event of a fire, there is another way to get out of the basement.

How to turn a basement into a living space

The ease of creating a basement room will depend on whether or not your property has an existing cellar and on the intended usage and size of space required. Typically, existing cellars tend to have limited headroom – insufficient for accommodation – and only extend under part of the area of the property. So even with a lower-ground floor in place, excavation work will be necessary.

Though the Building Regs do not stipulate a minimum head height, except at stairways, if the space has noticeably less headroom than the rest of your home, it won’t add as much value to the property.

Bear in mind that adding the waterproofing, underfloor heating and some flooring finishes will affect the available headroom by requiring the floor to be raised. A specialist basement-excavation company can ensure the space is substantial enough to accommodate layers of waterproofing and the installation of new electrics, plumbing and the finishes you want.

A basement can be added without an existing cellar by use of specialist underpinning. It is also possible to add a basement under an extension while it is being constructed, but this too, may require the existing property to be underpinned. To avoid this, the basement must be kept at a sufficient distance away with its superstructure bridging back to the existing property.

Converting a basement yourself

Generally, basement conversions are not a DIY job, at least with regards to the main alterations. The reason is that, if you need to lower the floors to increase headroom, it may undermine the foundations, leading to instability or even the collapse of the supporting walls.

Further complicating matters, a submission for Building Regs will be required and you must show that adequate precautions have been taken to prevent moisture ingress into habitable areas. Although the waterproofing is a possible DIY job, again it’s probably best left to specialist companies because the ground conditions, external materials and the state of your walls and floors all have to be assessed. It may even require a pump sized to suit the project.

DIY is therefore best kept to finishing work rather than the main elements of construction.

How to light a basement conversion

A lightwell is the most obvious way of introducing light and ventilation into a basement. There are potential planning issues – notably within conservation areas – but there are design solutions for most situations. A small opening with a walkable grille over it can make a big difference to the feel of a basement space, and add real value. Larger openings can have a full or partial protective covering in toughened glass.

If space is limited, you could include a sun pipe that uses a glass panels and angled mirrors, to bring natural daylight down into your basement. Equally clever solutions include reflective light-ducts, such as and hi-tech fibre-optic cable systems. Special light fittings are linked via fibre-optics to a solar collector on the roof that will track the path of the sun and feed light down to the rooms below, all day long.

Glazed ceiling panels in the ground floor above are another way to bring down borrowed light. These can be particularly suited to mews houses or more contemporary properties.

Dealing with damp in a basement conversion

Building Regs require that all floors and walls below ground level are waterproofed, to stop damp entering a building’s structure. Water penetration can be tricky to control; it’s not unusual to find basements where two or three different systems have been applied – bitumen, cellar paint, render – but all have failed for one reason or another. Find the right solution for your basement with an assessment by a qualified contractor with a proven track record.

The three main methods to choose from are brush-applied tanking, a waterproof concrete shell and a textured membrane. An air gap is left between the original wall and the membrane, down which any water can pass into drainage channels to be silently pumped away.

Whichever you choose, products should be British Board of Agrément certified and you should receive an insurance-backed guarantee. Of course, adequate heating and ventilation will help prevent condensation forming.

Fitting out your basement

The key considerations for the first stage of the fitting-out process are space, comfort and light. Mains services will have been installed, determining the position of any sanitaryware.

It is imperative that any damp-proof membrane isn’t punctured, though, so plasterboard walls must now be fitted on a metal frame that doesn’t touch its surface. The ceiling will then be suspended from the floor joists above and the side and roof cavities can be used for the electrics and inset light fittings.

Basements are naturally well insulated by the earth surrounding them, but will require additional insulation to meet Building Regulations requirements. This will usually involve adding a thick layer over the membrane and, if underfloor heating is to be installed, then insulation will also be laid beneath the hot-water pipes or electrical elements.

Traditional radiators can also provide heating if you have redirected the plumbing at an early stage.

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