Basements can create significantly more usable floor space and as they are underground, there is not usually an issue in obtaining planning approval for them. They cost more than the equivalent space above ground because you have the additional costs of excavating the ground and removing the surplus earth from site (unless of course it can be redistributed around your site as part of your landscape design). You save the cost of the external wall finish but incur the cost of tanking and better drainage. Aside from these factors, the costs are broadly similar.