A structural engineer should ideally check new structural members or alterations to existing structural members. This can often result in a more economic design than relying on "rule-of-thumb" sizing of members. The Building Control surveyor checking the plan or the works on site may require calculations to show that the new structure is adequate, or that the work will not adversely affect the stability of the existing building.
Floor joists
Existing ceiling joists are not likely to be adequate to act as floor joists for a new storey after loft conversion. Therefore, you will need to fit new, deeper joists. Floor joists can be supported on load-bearing walls, or floor beams (see below).
Load bearing walls need to carry the loads directly to an adequate foundation, or to a beam, or other structural member, that will support the load. They will generally be masonry walls, but timber stud walls may be used (subject to the structural engineer's calculations).
Any opening at lower levels in a load-bearing wall (eg: a through-lounge) requires an adequate lintel over it. This may need to be exposed and checked by the Building Control surveyor.
Floor beams
Beams to support floor joists can be made of solid timber, Glu-lam timber, steel, or a combination of timber and steel (a flitch beam). The exact size will depend on the loads on each beam and its span, but for spans of 4m or more, it is usually uneconomic to use solid timber sections.
Roof structure
You may need to alter or upgrade the existing roof structure as part of the conversion. You may need to removed existing rafters, purlins or props to install the new floor or roof windows. The loads on the roof structure may increase (eg: a new plasterboard ceiling to the underside of the rafters).
Where you remove purlins or any intermediate props, you may introduce a new beam underneath, or increase the size of the floor joists, to carry the additional load.
In a traditional roof design, the ceiling joists or collars are securely tied to the rafters, and purlins act as beams, in order to prevent the rafters from spreading. If these are cut or removed (eg: when installing a dormer), then you may require a ridge beam. In this case, you should consult a structural engineer.