Cost breakdown by beam type
Single 3m RSJ supporting internal wall: structural engineer design £600–£1,200, beam supply (UB 203×102 typical) £150–£300, padstones £80–£200, installation labour £600–£1,200, making good and finishes £400–£800. Total £1,800–£3,500. Double beam goalpost (2 beams, 2 vertical columns) supporting a 4m opening at the back of a side return: design £1,000–£2,000, beams £400–£800, columns £200–£400, padstones £150–£400, installation £1,500–£2,500, making good £800–£1,500. Total £3,500–£6,500. Major 5–6m kitchen opening with two parallel beams (one over the existing wall, one over the new extension wall): design £1,500–£3,000, beams £800–£1,800, padstones and supports £400–£900, installation £3,000–£5,000, structural concrete and connections £1,500–£3,000, making good £1,000–£2,000. Total £6,500–£14,000.
What determines the cost
Beam span and load: longer span and heavier load = heavier beam (UB 305 or UB 406 vs UB 203) = higher cost. Access: beam delivered by crane through the roof or carried through the house? Crane lift £600–£1,500. Padstone complexity: simple concrete padstones £50 each; reinforced concrete padstones with shear connectors £200 each on heavy loads. Fireproofing: beams in habitable rooms require 30 or 60-minute fire protection — intumescent paint £40–£80 per m of beam, or boxed in plasterboard £60–£120 per m. Temporary works: needle propping the wall above while the beam is installed £400–£1,200 depending on wall height. Making good: re-plastering, ceiling repairs, kitchen reinstatement around the new opening.
How to budget steel correctly on extension projects
A typical single-storey kitchen extension in London has 2–4 separate steel installations: 1) opening through the existing rear wall into the new extension (typical single beam £2,500–£4,500 supply and install); 2) goalpost over the bifold or sliding door opening (often forms one continuous structural beam with #1, total £4,000–£7,500); 3) ridge beam in the new extension if it's pitched-roofed (£1,500–£3,000); 4) any internal opening where a wall is removed (£1,800–£3,500). Total steel cost on a typical 25m² rear extension: £6,000–£14,000. Get all the structural design upfront from one engineer (£2,000–£4,000) — fragmented design from multiple sources leads to costly variations during the build.
