Comment by swiftcoder

You are permitted to change whatever you like on your car, subject to a roadworthiness inspection by the relevant transit authority.

Cargo van -> Camper van conversions go through this all the time - you add/remove seats, add a lot of weight in the form of beds, water tanks, etc. add/remove windows, put solar panels on the roof... After those changes you have to take it down to the vehicle inspection, and they tell you whether or not your changes have been deemed acceptable to drive on public roads.