Basements

Converting a basement or cellar can be a great way of getting extra room in your house. But these conversions involve a high level of scrutiny from your local authority building control team. This is because of potential issues around:

• damp 
• structural safety 
• fire escape

Straightforward basement or cellar conversions can provide a good return on investment, especially if there are no structural changes required. For projects that require excavation and/or underpinning, the costs will be significantly higher. 

Location, scale, build quality and specification and other factors can push up your basement/cellar conversion cost. Things that might increase costs include:

• having to divert drainage, or if the area has a high water table meaning you need a constantly working pump,
• if your property sits on certain types of ground such as clay, sand, marsh or made-up ground (previously excavated material)
• a difficult to access site or if there’s nowhere to store excavated soil
• high demand for parking on your street so you can’t get permission for a skip

Substantial basement and cellar conversions can be extremely complex projects. A poorly thought-out conversion can reduce a property’s value, and in some cases compromise the structural integrity of your home and neighbouring properties.

Like all building projects, it’s important to plan your conversion very carefully and get the work carried out by an experienced contractor.

Design issues to consider

When you are thinking about whether your basement or cellar is suitable for conversion you should consider:

• structure - is there sufficient headroom? Remember ceiling and the floor treatments that you will have to install are likely to reduce the available headroom
• means of escape - how will someone escape in a fire and how does this impact the rest of your property
• drainage - does the space ever flood?
• access - what is the access  and is there space for a staircase?
• storage - will you have enough storage space if you convert your basement or cellar?
• suitability - will a basement conversion give you the accommodation you need or would you be better extending the property?
• ventilation - is there any ventilation to your cellar or could any be provided?

Permissions needed

You'll almost certainly need planning permission if: 

• you are making structural changes 
• you are altering the external appearance of the property
• the building is listed or in a Conservation Area

You should also check with your local planning authority if it requires a Basement Impact Assessment. Redbridge is one of a number of London councils that require a Basement Impact Assessment submission in the planning application stage. 

If you are making structural changes and share a wall with your neighbour, you may also need an agreement under the Party Wall etc. Act 1996. 

Building regulations and basements/cellar conversions

Building regulations apply to all basements and cellar conversions, as well as the construction of new ones.

Building regulations apply to: 

• structure
• damp proofing
• fire protection and means of escape in the event of a fire
• drainage
• ceilings
• heating
• ventilation
• thermal insulation
• electrical wiring

Structure

If you don’t have sufficient headroom in your basement or cellar, you can sometimes lower the floor. This is not a straightforward procedure. You’ll need to consider if: 

• lowering the floor will undermine the foundations of your house or your neighbours’ house foundations 
• the floor will end up below the water table as this could make the property more vulnerable to flooding

Specialist advice is recommended before considering lowering floors.

Damp proofing

As most basements or cellars are set below the ground they tend to suffer from damp problems. 

A number of systems are available for damp-proofing basement and cellars. Most use a waterproof render system known as “tanking”. This forms a continuous damp proof layer across the floor and up the walls. 

Stairs

Some basements and cellars already have good, stepped access to them. However, deciding if the existing stairs provide suitable access to a habitable room is a matter for professional judgment. 

If there are no steps, or the existing steps are inadequate, a new stair will need to be installed. The best location for the stair will be influenced by a number of factors including: 

• the layout of the existing house
• available headroom 
• if a secondary means of escape can be provided from the basement or cellar area

When a new stair is installed in a basement/cellar conversion, it must meet the guidance around pitch, rise, headroom and handrails:

• the maximum pitch for the stair should not exceed 42 degrees
• the maximum rise of each tread of a domestic stair should not exceed 220mm and the going should be at least 220mm.
• headroom must be 2m above the pitch line - there are no reductions permitted 
• the stair should have a handrail at least 900mm high. Any exposed edges of stairs or landings should have balustrading at least 900mm high. Balustrades should not be climbable, or have gaps that allow a 100mm sphere to pass through. 

Fire safety

All basements and cellars converted for habitable use need a suitable escape route in the event of a fire. 

Often existing light wells and windows can be upgraded to provide a secondary fire escape, as well as ventilation. These windows need specialist fire hinges which can be fully opened so you can climb up to ground level in the event of a fire. 

If you cannot provide a secondary fire escape, the staircase needs to end up in a fire protected hallway with a door direct to outside. 

Converting your basement or cellar is a good opportunity to review the fire precautions in your home. Mains operated smoke detection significantly improves fire safety in the home. Building regulations require that it should be installed where basements are converted for habitable use.

Drainage

Drainage needs careful consideration at an early stage in any basement or cellar conversion as it can be problematic, especially if you are including a sink, bathroom, shower room or new fixed appliances. 

New appliances like washing machines need to connect to the existing foul drainage system. But, these are usually above the basement level. The solution is to install a pumped drainage system that macerates the drainage and pumps it to the existing drainage system. You will need suitable routes for pipes to run to a point where they can connect to existing drains.

Ceilings

Adequate ceilings need to be part of your basemen or cellar conversion, for fire, thermal insulation and sound-proofing purposes. 

Plasterboard is probably the most common material used for ceilings as it offers good fire resistance and flame spread properties. If your basement or cellar, or the room above, is to be used as a bedroom you need to install an acoustic quilt in the floor void between the rooms (for sound proofing).

Heating

If you want to install heating as part of the conversion, the most effective solution is extending the existing central heating system. Your plumber heating engineer will need to check that your existing boiler and pump has sufficient capacity to serve any additional radiators.

Ventilation

Any new habitable rooms in the basement or cellar need to be ventilated. This can sometimes be a problem for basement and cellar conversions. If natural ventilation through windows is not practical a mechanical ventilation system needs to be installed. These will need to be approved by your local authority building control team.

Thermal insulation

Your basement or cellar conversion should provide an insulated envelope so that heat loss is minimised. As well as insulation, you should consider the efficiency of any services you install. 

Low energy light fittings should be used where possible. Any new heating systems should have high efficiency levels and suitable thermostats and controls. 

More information about energy efficient heating systems is available on the planning portal

Electrics

It is sometimes possible to extend existing circuits to a basement or cellar conversion but often new circuits and a new distribution board will be required. Get advice from a competent electrician at an early stage to see if your electrical system can be extended.