This is an interesting question to which there is no standard answer I am afraid, largely because the cost of construction will be so influenced by the engineering required on site.

From your description, I am assuming that the site slopes up and that your new driveway would lead to a basement garage underneath the bungalow. In circumstances like this, much of the structural work necessary to create the basement void is likely to have been included within your foundation design and, dependent on the prevailing soil conditions, the depths of these foundations may be quite substantial.

This being the case, the notion of excavating a large void to create garage space or even accommodation can be very cost and space effective. However, in doing so your side and rear foundations will need to extend deeper and the above foundation masonry designed to act as a substantial retaining wall to hold back the surrounding soil. Waterproofing and drainage detailing is also important to ensure the space is kept dry. It’s impossible to give you costs for carrying out this work without a site report and drawings.

However, a conventional detached double garage is likely to cost around £15,000. That kind of budget should cover the elements of an underground garage as described above. It might be a good idea to try to incorporate a rear internal stairwell and possibly a boot/utility room as well, depending upon the space available, but obviously this will increase costs.

Tim Doherty, Dobanti Property Consultants