There are few house designs which will not require the input of a structural engineer at some point. Although span tables are available for structural timbers such as floor, ceiling[for the purposes of part B of the Approved Documents] - A part of a building which encloses and is exposed overhead in a room, protected shaft or circulation space. (The soffit of a rooflight is included as part of the surface of the ceiling, but not the frame. An upstand below a rooflight would be considered as a wall.) and roof joists and loading tables are available for standard steel lintels, that is pretty much where it ends because most of the other structural calculations will be unique to the building. At the time of submission of drawings to the building controlthe local authority department which deals with the Building Regulations department, the building inspector will want to see calculations for anything non standard.
If you are employing an architect it is normal for them to liaise with a structural engineer and get it all sorted out. Larger practices have in-house engineers. The same will apply if you are going through an architectural technician. If you want to do the design yourself you will need to contact a structural engineer directly. Be aware that if you want to do anything structurally complicated with timber it is best to check with the engineer that they do that sort of work. Timber structures may present challenges and opportunities; masonry is traditionally quite well understood and building inspectors can assess designs relatively easily whereas timber is often relying on complex configurations of quite slender structural members which need detailed calculations.
Engineers will also advise on matters like the bearing capacity of land for foundations, design of retaining walls, protecting existing drains etc. For anything out of the ordinary it is worth getting an estimate for their work beforehand as rates can vary considerably
See the Institute of Structural Engineers web site to find local practitioners.