It is common knowledge that buildings should be constructed with good envelopes and sufficient masses. However, U.S. building codes make no provision for a building's thermal mass. This may result from that the conventional heat balance design—which assumes constant indoor air temperature, any heat imbalance resulting from corresponding envelope heat loss or gain and internal heat gain is accounted for by HVAC equipment—aims for HVAC selection, not for determining required building thermal qualities. We propose the concept of building thermal autonomy and a two-step process assumption-based design method. Thermal autonomy is a building's capability of keeping its indoor temperature sans HVAC equipment within a prescribed temperature range. The first step of the process assumption-based design method aims for the determination of building thermal qualities for a thermally autonomous building. Our finding shows that optimum slab thickness for a building's ceilings and floors is 25-cm and recommends 10-cm thick concrete slabs for its envelope walls. More interesting is the finding that climate-specific factor of diurnal temperature amplitude should be taken into consideration in the envelope design: a maximum WWR (Window-to-Wall Ratio) is given as a function of diurnal temperature amplitude.