Introduction

A basement development can add more space to a property than a loft conversion, as it usually opens up a whole new level with an area equivalent to that of the entire ground floor, all of which has full head height of 2.4m (7ft 10½in).

However, excavating a basement is a complex piece of engineering that produces more mess than a loft conversion and generally costs more too.

As basement developments are supported by solid ground and concrete, they are suitable for a wide variety of uses including heavy loads such as car parking, swimming pools and gymnasiums, but of course also lighter purposes such as games and media rooms.

Planning permission

Building regulations

All basement developments will be subject to building regulations and will need to be certified by a Building Control Officer. Regulations applying to ventilation, damp-proofing, fire escape, energy efficiency, electrics and water supplies will be given particular attention.

• Regulations & permissions for extensions

• Foundations need to be adequate to support the structure of the whole house, including the new basement.
• Occupants of the basement must be able to escape from a fire by means of an independent staircase or fire egress window.
• The walls of the basement must be suitably damp-proofed.
• Adequate ventilation must be provided to the basement by means of an open staircase, a window or airpipes, especially if the basement houses a bathroom, bedroom or kitchen.
• All new glazing in the extension must have a U-value of 2.0 or less.

To overcome any concerns of subsidence, if your home is terraced or semi-detached or the excavating of a basement will in anyway approach a neighbour’s boundary, you will invoke the Party Wall Act 1996, and you should be sure to gain written consent from your adjoining neighbours for work to proceed.

• The party wall act

Your current property

Older style properties, such as Victorian (c. 1837 - 1901) homes, are most suited to the addition of a basement as they have suspended timber floors and main drains that are positioned to the side of the house, although their foundations tend not to be deep enough and therefore need to be underpinned (as will any adjacent foundations of neighbouring properties).

Basements can be excavated beneath modern solid-floored homes, but at a considerably higher cost due to the added complexity of adding joists beneath the existing ground floor as well as underpinning the whole basement.

If your home already has a cellar, it will be significantly cheaper to excavate sideways from this, than if you have no existing subterranean level where digging will need to be much deeper in order to support the existing ground floor.

The water table

The key to success in creating a useful basement is making it completely waterproof. Before embarking upon a basement conversion, you should investigate the level of your local water table, as the higher this is, the more costly and complex the waterproofing process is. Contact your local water supplier to establish your water table level.

• Water companies' contact details

There are two ways to waterproof a basement, both of which create a ‘room within a room’ to isolate the inner wall from the outer wall with an air cavity.

‘Tanking’ uses a metal skin, while the alternative method uses waterproof membranes to line the inside of the basement and both of these surfaces can be plastered over. A cavity is left between this inner lining and the outer walls of the basement so that water cannot cross, instead trickling down the outer walls into drainage sumps from which it is silently pumped away.

Further information and useful links

• Guide to conservatories
• Guide to loft conversions
• More building works topics
• Making structural changes to your property
• Walls & ceilings topics

• Find a Basement conversion specialist near you
• Find a Builder near you