And for the footage I can ask the next door neighbor.

Currently renovating, will have 3 bed, 2.5 bath and double the square footage when completed.

New Construction ADU, Attached: Parcel size less than 1 acre: 850 square feet (studio or one bedroom), 1,000 square feet (two or more bedrooms), or 50 percent of primary dwelling habitable square footage, whichever is smaller.Parcel size greater than 1 acre: 50 percent of primary dwelling habitable square footage.

Next to a tennis court, basement space for additional square footage to increase value, currently in live-able condition, multiple comparables.

The problem today: most appraisers used by traditional lenders would give ADU square footage a second-class treatment than the main house (as some of your mentioned above) since there aren't that many comps and they're trying to be conservative. 

You can extend your house or increase the main house square footage where you add a room with an door for entrance/exit.

Some places require space for a primary leach field and a backup or secondary leach field, so you will need to make sure you have enough square footage of suitable soil to support a septic system for a house of the size you are wanting to build.

You should ask questions like: Is there unused square footage that I can use to add bedrooms?

However many local municipalities will have restrictions (generally based on square footage) and your homeowner's insurance may as well.Absent either of those, I would go with a "reasonable" restriction. 

I would add is your plan contiguous with neighborhood- similar square footage and height?