Architectural technicians
These are the people who know the nuts and bolts of building. They may work for architects or they may be in a separate business, sometimes linked with a builing company. If you go to them with a design you have carefully worked out they can usually knock it into shape ready for getting planning consent, building regulations approval etc. Don’t expect them to create a magnificent work of art, that’s not what they are trained for. Architects can do that - on a good day.
It’s best to get an estimate from them before you start and further estimates as you go along if you want to change your mind about something because making changes during the course of the work can be expensive.
