Converting a basement to living area adds very little to the actual heating and cooling demands of your system. It is not like an addition, where windows and doors and external walls are added. Chances are your current system is already dealing with whatever heat loss or heat gain the cellar generates.
But there are a few considerations:
- A gas furnace requires combustible air to operate properly. If you build a utility room around the furnace, combustible air must be provided.
- If the basement is below outside grade, it will not be readily effected by outside temperature changes, and the spring and autumn requirements will differ from the rooms above. The best solution, in this case, is to independently zone the cellar from the rest of the house. In fact, the zoning must be Total Zoning, which will allow the basement zone to change the furnace mode from heat to cool despite what the upstairs zones are set on.
- Exterior walls should be insulated. And if there is insulation in the cellar ceiling, I would also remove it, especially if a the insulation has a vapor barrier. You might be able to use it on the walls.