Basement house extensions are they worth it?

Excavating under the existing house really is the last resort. Not only is it very expensive it is also fraught with dangers of collapse and settlement causing all sorts of other problems throughout the rest of the property or the adjoining neighbours.

When homeowners engage in this type of work for a house extension in order to create more living space, you really know that they are desperate and have more money than they know what to do with.

Basement home extensions only really ever make sense financially when the property is within a high density urban environment that has super high property values and there is no other alternative on the site to create the additional floor space.

Extravagant examples of the current craze for going underground in the ‘City bonus belt’ of Kensington, Notting Hill, Holland Park and Knightsbridge abound. Typically, basement projects range from installing squash courts, garages and gyms. Then there is the double-height room with the space for the owner to spring off a high diving board into a pool.

You need to pay from about £300 a square foot and employ a battery of expensive specialists a solicitor, surveyor, engineer, damp-proofing expert and builder to carry out the work. However, once you take into account removal costs, estate agent and solicitors’ fees and Stamp Duty (totalling about 7% of a house’s value), it could make sense to build a sub-basement rather than move to a bigger house. ‘Rooms that don’t need windows, such as a gym, Turkish bath, wine cellar, media room and pool, are best suited to being hidden under the surface of the earth.

The home basement extension will need as high performance DPC and DPM for the basement waterproofing system which is usually a high performance polymeric material designed to suit most applications. Good tanking or waterproofing systems should normally be applied externally to the supporting structure so that the water pressure does not blow it away from the surface. However, in most situations other than for detached property's with good perimeter space this will not always be possible.

Basement Conversions for a home extension on the other hand are a little easier and far less costly. This is where an existing damp basement or celler is upgraded with good access & tanking arrangement to create a habitable room that would otherwise go unused.

Incorporating a basement within a new build home extension is another alternative that is very possible but will add significantly to the construction costs. Consideration needs to be given to the access, means of escape, fire precautions & lighting. The sometimes simply end up as a gym or other recreation area requiring mechanical ventilation.