A cellar conversion is one of the few home improvements where an hour with a tape and a torch tells you most of what you need to know. Four things decide whether the project is a fit out or a major structural job, and they can all be established before anybody is paid to have an opinion. Doing that first is what stops a cellar conversion becoming an excavation.
Measure the headroom under the joists
This is the question. A cellar with enough clear height for a habitable room needs drying, insulating, lighting and finishing. One without it needs the floor lowered, which means underpinning the walls in sequence with temporary works and a structural design, and that is an order of magnitude more work. Measure to the underside of the joists, not to the underside of the floorboards.
Look at how the water is getting in, and when
Cellars are wet in different ways: a steady damp through the walls, water appearing after rain, or standing water in one corner. Each points somewhere different, and a defective gutter or a blocked gully outside is a far cheaper fix than a waterproofing system. Look outside before you look inside.
Find the intakes and the services
Gas meter, electricity intake, stopcock, and often a run of soil pipe. Relocating a utility intake is the utility company's job, on the utility company's timescale, and it can add months rather than money. It belongs in the plan at the start.
Establish where the light and the escape can come from
A light well at the front, a pavement light, or a lowered area at the rear are how a cellar becomes a room. Without one of them the space can be a store, a plant room, a utility or a gym but not a bedroom, and pretending otherwise produces a room that fails building control and never gets used.
Questions people ask about cellar conversion
Is my cellar dry enough to convert?
Almost no British cellar is dry enough as it stands, and almost all of them can be made dry with a designed system. The question is not whether it is dry now but what it will cost to keep it dry, which is a design question rather than a treatment.
Do I need planning permission to convert a cellar?
Converting an existing cellar into habitable space often does not need it. Excavating to enlarge it, or forming a light well or an external stair, frequently does, and some councils control basement work specifically.
Will a converted cellar be cold?
Only if it is not insulated, which is easy to forget in a space that feels temperate. The walls are in contact with the ground, so they need insulation on the room side of whatever waterproofing system is used, and the floor needs it too.