Garage conversion into Annex approved - Building control guidance required

So I have a detached garage for which I have been given planning permission to proceed with the conversion. The roof needs to remain as is but I can add 3 windows, 2 roof windows.

Getting quotes from a single builder is becoming very cost prohibitive and all the quotes I got to do the work above + insulate and add a bathroom is costing £60k.

I believe I can take this down significantly by hiring individual contractors and plumbers, sure it is more hassle and will take more time but I can live with that. For electrical work I will use a NICEIC electrician so he can issue his own Part P certificates under the competent person scheme.

How would I handle building control in such a case? Can I just agree stages of work with them and call them in periodically for inspection?

Appreciate any thoughts please from the experienced folks.