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 building 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 consentthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place) which are all about whether the building is properly constructed). see more on Planning, 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.