The 3200 sqft is presumably a multiroom home. If so, a central system would work best. The 800 sq ft unit could use a minisplit if it's 1-3 rooms. Furnace/AC vs heatpump really depends on the gas and electric rates where you live.

You need to contact 3-4 HVAC companies for estimates (and get them to run the gas vs electric numbers).

The first is about 3200 sq ft and the other is 800 sq ft ADU in Silicon Valley, California where the weather is mild and does not go to extremes.

Initial research seems to indicate that the main unit will be good with 5 T and ADU with 1.5T unit.

A size of 5 tons for 3200 sq ft, and 1.5 tons for 800 sq. feet seem oversized for new construction in a mild climate. How did you come up with these sizes? I believe California requires a Manual J load calculation. Your HVAC contractor should give you a copy of the calculation and you should review it to see it it makes sense to you.

Is heating with natural gas an option? If not then heat pumps would be my recommendation.

Hi, bunty247,

Your architect should have a mechanical engineer or qualified HVAC design professional whom they work with regularly. They can help you make an informed choice between the various alternative systems. They'll also be aware of any local codes and issues e.g., whether your locale has prohibited natural gas service to new construction or is likely to do so in the near term. You should also check with your local electric utility regarding rebates and incentives which might favor one system over another.