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Basement Conversion

Guide to Converting a Basement

We've compiled a comprehensive guide to converting a basement. Converting and extending a cellar can provide valuable extra living space without drastically altering the exterior of your home.

The one thing that most people want in their home is more space, and increasingly homeowners are looking to find this by converting and extending their cellar to create a basement beneath their existing property. Unlike loft space at the top of the house, which lends itself best to creating additional bedrooms, a basement is located close to the main living areas and access, and as such has a more flexible range of uses. A basement is the ideal location for additional family living space, such as a playroom or home entertainment room. It is also a great place to relocate the utility room, boiler and storage, freeing up more valuable above-ground space. Alternatively, a basement can have its own external entrance and provide a self-contained unit, ideal for use as a home office, annexe, or a even an entirely separate new dwelling.

Planning Permission

Converting an existing cellar beneath a dwelling from a storage area to habitable space involves only a ‘change of use’ and so does not require planning permission. Reducing the floor level of a cellar to improve the ceiling height will enlarge the volume of the property and is therefore treated as an extension and so may need planning permission. Under certain circumstances, however, modest extensions and alterations can be undertaken without the need to make a planning application under what are known as Permitted Development rights: for more information visit

Planning policy on basements varies but it is very difficult for a local authority to find reasonable grounds for refusal, especially if the work does not significantly alter the building’s appearance.

Building Regulations

The creation of a new habitable basement will require Building Regulations approval regardless of whether it involves a change of use of an existing cellar, or the creation of a new or larger basement through excavation. The Building Regulations are statutory minimum construction standards that ensure buildings are safe, hygienic and energy efficient. The renovation of an existing habitable basement, or the repair of a cellar that does not involve a change of use, i.e. from storage to storage, is excluded from the Building Regulations.

For guidance on meeting the Building Regulations, get a copy of the Approved Document — Basements for Dwellings, which includes all of the relevant regulations.

It is best to make a Full Plans Application for a cellar conversion, rather than to follow the Building Notice procedure, as this allows all design details to be resolved in advance of the work.

If the proposed works affect a Party Wall – e.g. if beams are to bear onto a Party Wall; the wall is to be extended, altered, underpinned; or if excavations are to be carried out near to a Party Wall – the owners and leaseholders of both the building within which the proposed basement works are to take place and those of adjoining properties must be informed. The relevant legislation is the Party Wall etc. Act 1996. Log on to for more information.

Specialist basement contractors offering a design and build service will handle planning, Building Regulations approval and any Party Wall agreements as part of their service.

Duration of the Project

If there is access directly onto the garden or highway for the removal of soil and your existing ground floor is suspended timber, you can probably continue living in your home whilst work is under way. If the ground floor has to be removed and rebuilt because it is concrete, and all of the spoil has to be carried through the house, you will almost certainly have to move out.

A simple cellar conversion can be completed in a matter of weeks. Converting and extending the cellar beneath an entire house, involving underpinning of the existing structure, is likely to take several months.

Cost Versus Value

Whether a cellar conversion is financially viable will depend on the cost of the work relative to local property values. It almost always makes financial sense to add lower ground floor space in high-value areas, such as central London and other sought-after urban centres. Elsewhere the potential to add value needs more careful analysis. Estate agents should be able to give you an indication of how much space is worth per square metre in your area. Bear in mind that good-quality, well-lit basement living space with access directly onto the back garden will be worth considerably more than dark, converted cellar space with compromised headroom. It is also important to take into account the ceiling value for the location: remember, most suburban areas, and especially estates, have a maximum value that is very difficult to exceed no matter what improvements you make.

The cost of conversion: Providing there is adequate headroom, turning an existing cellar into extra living space would cost around the same as for a simple loft conversion.  It is only when you have to start lowering the floor level to increase headroom, involving digging out the ground beneath the house and underpinning the foundations, that the work starts to get expensive. These costs will be higher if: the existing ground floor is a concrete oversite slab rather than traditional floor joists suspended from the foundation walls; if you have to divert the drains; if the ground conditions are difficult, such as solid rock, clay, sand, peat or marsh; or if the local water table is high, necessitating constant pumping during construction. Access for removing the subsoil is another important consideration that will affect costs.

Factors that add to costs:

  • The need to divert drains beneath your house
  • Your home has solid concrete rather than timber subfloors
  • Your home sits on difficult ground conditions such as clay, made-up ground, sand or marsh
  • The local water table is high, necessitating constant pumping
  • Access to the site is poor

Waterproofing Your Cellar

Tanking: Waterproofing below ground level is often referred to as tanking’ the application of a layer of waterproof material directly to the structure. This is usually a cementitious waterproof render system on the walls, typically applied in several layers, linked to a waterproof screed on the floor. Tanking can also involve a sheet membrane, asphalt or other liquid-applied waterproofing material. Tanking is also required to withstand the external water pressure around the cellar (hydrostatic pressure). The pressure from the water table around a basement can be enormous and unless the tanking is very securely fixed to the substrate, it can fail. Hydrostatic pressure will force water through the tiniest fault very rapidly and once a leak occurs it can be very difficult to isolate and repair.

Cavity Membranes: Cavity drain membranes are an alternative. The membranes are used to create an inner waterproof structure in the basement or cellar, behind which is a cavity (created by the membranes studded profile) that is fully drained, so any tiny leaks in the outer structure are diverted harmlessly away via a drain. By constantly draining away any small leaks there is never any water pressure against the inner structure. Several reports consider cavity membranes to be the most reliable way to waterproof a basement.

For a FREE design consultation contact Milestone.

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