Once you start to accumulate bikes and have been riding for awhile, you usually start to become your mechanic doing the primary care on your bikes. You end up doing the work out of necessity, pleasure and for cost benefits, and who better to take care of your baby than yourself. Having a functional space to do your work is important, and it needs to be comfortable and roomy enough to perform the maintenance. The floor space (square footage) must include enough room to maneuver the bikes around, incorporate bike stands to hold them during maintenance, storage for parts and tools, and a workbench for staging. Depending on your skill set and needs, you might do most of the work yourself and just leave the most difficult tasks to your LBS mechanics (aka the professionals).
My home bike shop is located down in the large storage room in the basement. It is roomy and comfortable, and warm in the winter, cool in the summer, and I can get the room dirty if I want. Depending on where you live, you might be able to use a portion of your garage as the workshop, but it's not an option for me due to the arctic-like temperatures throughout the wintertime.