If I modify the seating capacity of a bus, is a compliance certificate necessary?

Yes. There are many factors to be covered off by such a bus modification, including: axle weight distribution, investigating what bus rules are applicable to the modifications & the service operations for the bus, seat anchorages, seat spacings, if seat belts are applicable, aisle width, emergency exits compliance, and technical specifications. The vehicle insurance cover could also be void if modifications are not certified.