Based on your question, I presume you are planning to build in an area where heating is normally required. The key is careful design and cost estimations. For

Based on your question, I presume you are planning to build in an area where heating is normally required. The key is careful design and cost estimations. For example, smaller houses are cheaper to build and to adequately insulate. A heat-pump is more effective for heating than a gas fired boiler. You can store heat in thick concrete flooring, but it costs more than regular flooring, etc.