It isn’t really possible to answer this question without looking at the plans. If your drawings show a standard trussed rafter system it is likely that no allowance has been made for a traditional cut-room alternative. Fundamental issues need to
be considered, such as the availability of load-bearing internal walls right down to foundation level or steel beams in certain locations where a wall is not available. And given that you are about to start work on the roof, it’s unlikely that you’ll be able to resolve this issue without delaying the build.