RetireJapan

I am a home owner. I have reviewed the existing plans and believe that the house is at maximum footprint but not maximum floor area.
I am considering filling half the carport, which sits under the second floor, to make an additional room as a home office.
We have no use for a car and regularly use share car rental places nearby and see this as a better solution for our lifestyle than buying an own car for such intermittent use. The house is approximately 5-6 years old

This would change the total floor area from 100m2 to approximately 110m2.

Someone suggested changing the floor area would increase certain taxes.
Is there any where I can look to see what the financial impact might be?

I'm not sure, but any construction/'reform' company should be able to give you advice about this (as part of getting a quote to do work without signing a contract).

I also changed the title for you, as residential tax is usually 市民・県民税 while property tax is 固定資産税 which is what changing your house's floor area might impact.

Anyone else?

When we had our assessment for property tax, square metres of the building definitely formed part of it, along with materials/windows etc, so I would think the work you are thinking of doing would add a bit on to your property taxes.

Your property tax would certainly go up a bit, but really not much. Maybe what you're paying now plus 10-20,000 yen a year?
It's probably not enough to really worry all that much about. You'd save more by not owning a car.

IIRC, your property tax is based on the value of the house as of Jan 1, so if you wait until early 2021 to finish the extension, you wouldn't see any increase in property tax until 2022.