A lot of people have hobbies that have the ability to take up a lot of space, so it makes perfect sense to convert your unused basement in to a hobby room. The most cost-effective option is to keep the basement the same shape as the ground floor footprint, as no further foundations will then be required to support the house. Where deep trench-fill foundations are being specified because of ground conditions, going a little deeper and excavating a basement is worth exploring. Tall ceilings improve the quality of space, and larger rooms generally require a taller ceiling to balance out the proportions.
Unlike above ground, where taller ceilings push up the ridge height, potentially creating a conflict with neighbours and the planners, with a basement all you need to raise the ceiling height is to dig deeper. There is no minimum ceiling height for basement ceilings under the Building Regulations, but a practical minimum height is 2,400mm, and the taller the ceilings the better.
If the basement is to form a separate dwelling, then most local authorities will apply a minimum ceiling height when determining the planning application of 2,300-2,400mm. The ceiling will need to accommodate services for lighting, ventilation and plumbing, and so a service void will be required, the depth of which will be dictated by duct sizes, choice of light fittings, and whether you opt for a suspended precast concrete floor or suspended timber floor.
Engineered joists with an open web, such as Posi-Joist, are likely to be the most space-efficient option as services can be run within the floor structure, keeping down the overall build up, and therefore allowing maximum ceiling height with minimum dig depth. If the ground is being excavated or the basement extended, then existing walls are likely to need underpinning and the floor structure supporting using timber or steel beams.
Removing waste from bathrooms, kitchen sinks and washing machines relies on gravity, but in a basement it is more than likely that the sewer level will be above the slab level and so all foul water will have to be collected in a sump (which will need to be sealed and vented) and pumped up into the sewers (or off-mains drainage tank). However, the basement layout still needs careful thought and all facilities requiring drainage connection will need to be positioned to allow sufficient fall to the sump of at least 12.5mm every 1m (1:80).