When SE input is mandatory
Foundation design (Building Control requires calculations for anything beyond NHBC-standard strip on firm ground). Beam and lintel design for any opening larger than the standard tables. Retaining walls. Roof structural sizing on anything non-standard. In practice every KDR needs an SE engaged from Stage 3.
Fee bases
Lump sum is the dominant model on single dwellings: £2,500-£5,000 for a developer-spec replacement, £5,000-£8,000 for mid-range or premium, £8,000-£18,000 for architect-designed with bespoke spans, cantilevers, or basement. Percentage fees (1-2% of structural works value) are used on larger or commercial projects.
Why IStructE matters
The Institution of Structural Engineers chartered status (MIStructE, FIStructE) is the recognised standard for residential structural design. NHBC, LABC, and most home warranty schemes require calculations from a chartered engineer; warranty providers may refuse sign-off without it.
Overlap with the architect
The architect typically appoints and coordinates the SE under a sub-consultancy or direct-appointment arrangement. Direct appointment by the client gives the SE independent professional liability but adds coordination friction; sub-consultancy under the architect simplifies but bundles the fee into the architect's invoice.