Generally a pre purchase building inspector will inspect and report on the property as a whole. Meaning looking for and reporting on defects in roofing exterior and interior, sub floor areas, interior and exterior and the yard and fences. Possibly even electrical, plumbing, mould or asbestos if qualified in the relavnt area.

A structural engineer will inspect and report on areas that are crucial to the structural integrity of a property. Items such as load bearing walls, piers and stump and footings and load bearing timbers in the roof or sub floor area. A structural engineers report is a more in depth and technical look at these items wheeras a pre purchase building inspection is an all round view of the property.

A structural engineer inspection and report will be more expensive than a general pre purchsae building and pest inspection as they are highly qualified and the report will be more technical.

Its not uncommon for a pre purchase building inspector to recommend to the client to engage a structural engineer if structural problems are encountered during a pre purchase property inspection. The engineer will then carry out an inspection and provide a structural report for the relevant area, EG: a badly cracked wall, termite eaten supporting timbers in roof or sub floor, sinking footings in the sub floor area etc.

You could consider a pre purchase building inspector as an all rounder like a GP. Who will recommend a specialist if issues are found they are not qualified to address will recommend a specialist in the particular area. EG: structural issues, electrical or plumbing faults, mould or asbestos.