Have an older home with 100 amp service; 2500 square feet in a U-shaped footprint. Need to replace ancient HVAC system and also plan for future splitting of home into a duplex. City and fire marshall requires one meter when there is no firewall until zoned as duplex. Eventually, all will be electric, two kitchens, heat-pump with strips. Do I need a 300-400 amp service, or what size will eliminate having to do this again? If using a bigger amp service, can I set up two main breaker boxes with 200 amp for the expected three bedroom unit and 100 amp for 1 bedroom unit? Trying to keep costs down and yet plan for future when 2 car garage would be converted into two bedrooms and a laundry room. Suggestions are appreciated.

A residential 400 amp service can be installed, called a 320 amp service. This can feed two 200 amp panels, or one 200 and one 100 if so desired.

If this size is correct is is unknown until a service calculation is done. This needs to be done by an electrician, and the final outcome will depend on the square footage of livable space, and a list of all major electric appliances, air conditioning, electric heat.

If you can provide all of this detail, we can do the calcs here.