I have 1100sqft home (measured on the outside). It is single level with full basement (each 1100sqft). I know the basement is not taken into account when sizing heating system but it is used as separate 4 bedroom rented apartment so maybe I should add the footage? Anyways, I would like to get new HVAC system for the house since now there is no A/C and furnace is probably about 30 years old. However I am planning to add another floor to the house in next 2 years and if I get new complete HVAC system I wish it could accommodate the the addition. Is there any way I can do it and what size of HVAC system I would need before and after the addition? Through some research I have done so far I have read that I could use two separate heating systems or do a bay-pass if I decide to go with larger system. Though either way is possible solution I wonder why I can not just get bigger system now (like 2 tones)? One place told me the system could brake if it is too big. I understand that oversized A/C will not perform very well but maybe somebody can shine some light why oversized HVAC system could brake?