What sort of heaters are you using?
Electric tubular heaters with guards (mounted higher up and well out of reach) would be the only ones I would consider. Otherwise, good insulation in the walls, and a dry, draught-proof area with a box of hay to snuggle into with another bunny or two should be fine. In really cold weather (-5'C), I use Snugglesafe heatpads in the hay. Healthy rabbits that have lived outside during the Summer / Autumn should be fine in the Winter as they will develop a much thicker coat. Often they will choose to sit outside in the worst weather despite everything you provide.