Can my builder use 'rule of thumb' beam sizes?

No — Building Control requires calculations for all structural members. Rule of thumb risks over- or under-sizing; legal exposure if collapse. Always engage Chartered Structural Engineer; budget £950+ fee even on small project. Builder's experienced opinion useful as sense-check but not a substitute for calc.

Do I need a structural engineer for a single-storey kitchen extension?

Yes if removing existing external wall to open up to extension — almost always the case. Engineer specs beam carrying first-floor + roof loads above. Even simple side-return with bifold doors: lintel/beam over the new opening sized by engineer. Building Control will reject submission without calcs.

Can architect do structural calcs?

Most architects don't — separate professional discipline. Some larger architecture practices employ structural engineers in-house. Default arrangement: architect designs scheme + appoints structural engineer (often via panel of preferred consultants). Verify in architect's appointment what's included + what's separate.

Structural Steel Design Cost London 2026 | Builderr

How Much Does Structural Steel Design Cost on a London Extension?

London structural steel design cost £950–£3,850 for typical side-return or single-storey rear extension; £2,850–£8,500 for double-storey, complex loft, or large open-plan removals. Fee covers structural calculations, Building Control submission package, fabricator drawings, site visit. Steel supply + installation separate: £1,850–£3,500 small package, £5,500–£12,500 large open-plan. Use Chartered Structural Engineer (IStructE MIStructE/CEng minimum). Foundation design separately fee'd where new pad/raft required.

Scope of structural engineer work

Pre-design: site visit + measured survey of existing structure (cost £350–£950 if separate). Detailed design: structural calculations to Eurocodes (EN 1991–1999 series), member sizing (universal beam, parallel flange channel, hollow section, column), connection design (welded/bolted; bearing onto padstones), deflection check (span/360 typical for domestic), lateral stability check. Drawings: structural plans + sections + connection details + member schedule + bar bending schedule (if reinforced concrete). Building Control submission package: calculations + drawings + design statement + load assumptions. Liaison: respond to Building Control queries, fabricator queries, site queries. Site visits: typically 1 at first-fix (verify install), additional visits per agreed contract. Sign-off: Engineer's Final Certificate at completion for warranty/lender.

Fee scales typical

Single side-return or simple rear extension (1–2 beams supporting existing wall): £950–£2,250. Larger rear extension with full open-plan ground floor (3–6 beams + 1–2 columns + foundation design): £1,850–£3,850. Double-storey rear extension (more beams + first-floor framing + larger foundations): £2,850–£5,500. Loft conversion (steel beam install at first-floor ceiling level + ridge beam if mansard): £1,250–£3,500. Combined extension + loft: £3,500–£6,500. Whole-house renovation with structural alterations: £4,500–£12,500. Basement/underpinning: £4,500–£18,500 + specialist input. Wide-span open-plan (8m+ unbroken): £5,500–£18,500. Fees usually fixed quote or % of structural works value (3–8% typical). Compare quotes: ensure scope covers BC submission + 1 site visit minimum + as-built drawings + Engineer's Final Certificate (some firms charge extra).

Steel supply + install

Steel package separate from design fee. Typical extension package: 2× 152×152 UC + 2× 305×165 UB + connections + padstones = 0.8–1.2 tonnes steel supplied + installed £1,850–£3,500. Large open-plan ground floor removing 2 internal walls: 4–6 beams + 2–3 columns = 1.5–3 tonnes = £5,500–£12,500. Loft beams: 2× 178×102 UB typically = 0.4–0.6 tonnes = £1,250–£2,850. Fabricator drawings produced from engineer's structural drawings (may be additional cost £350–£950 if engineer doesn't provide). Steel erection: requires crane or telehandler for beams >150kg; small site access constraint (no front access) may require dismantled steel + on-site welding (specialist + significantly higher cost £8,500+). Padstones cast in advance (concrete cast in masonry pockets, 7 days cure before steel landing).