How many BTU do you need to heat a 3500 sq ft house?

110,000 btu

Since British Thermal Unit deals with volume rather than area, we need to know the height of the ceilings. The original answer probably (and sensibly) assumes standard 8' ceilings. Hate to sound like a nitpicker, but many of today's homes have at least one room with a "cathedral" or vaulted ceiling. My previous home had "Saltbox" architecture; at one place in the great room (which comprised 1/2 the house's total square footage) reached a height of 42' sloping to 8' in front and 35' in the rear. The total square footage of the house was ~1700, where the cubic footage must have been almost 3 times that of a standard 1700 sf house.