If you are knocking through to open up your kitchen or supporting a new rear extension, the honest answer is that no one can tell you the beam size without doing the calculations. What we can do is explain how sizing works, what beams are commonly used in London homes, and what it typically costs, so you know exactly what to ask for and what to expect.
Every steel beam is sized for the specific loads it carries. A beam under a single storey flat roof does very different work to one holding up two floors of brickwork and a loft conversion above a wide kitchen opening. The calculation looks at the span of the beam, the width of floor and wall it supports, the weight of the materials above, and any point loads such as another beam or a padstone landing on it.
The engineer also checks deflection, which is how much the beam bends under load. A beam can be strong enough not to fail yet still flex enough to crack plaster or make floors feel bouncy, so deflection often governs the final size rather than strength. This is why a beam that looks oversized to a builder may be exactly right on paper.
Most domestic work in East Ham and the surrounding boroughs uses Universal Beams, written as depth by width by weight per metre, for example 203 x 133 x 25 UB. Terraced and semi detached houses here tend to have similar spans, so certain sections come up again and again, though the final choice always depends on the calculations for your house.
As a rough guide only, a 3 metre opening in a load bearing wall supporting one floor might come out around a 152 x 89 or 178 x 102 UB, while a 4 to 5 metre opening carrying floors and masonry above often needs something in the 203 x 133 to 254 x 146 range. Wide open plan rear extensions with a 5 to 6 metre clear span, especially where the beam also picks up an upper floor, can require 305 deep sections or a pair of beams bolted together.
Removing a load bearing wall or forming a new opening needs Building Regulations approval, which in this area means Newham Building Control or an approved inspector. They will ask for structural calculations prepared by an engineer, showing the beam size, the bearing length at each end, usually 100 to 150 mm minimum, and the padstones or spreader plates that stop the beam crushing the brickwork below.
If your house is a terrace or semi and the beam bears on a shared wall, the Party Wall etc. Act 1996 usually applies as well. You will need to serve notice on your neighbour before work starts, which is separate from building control but often overlooked until it delays the job.
Structural calculations for a straightforward beam usually cost somewhere in the region of £250 to £500, with more complex extensions involving several beams and posts costing more. The steel itself is often less than people expect, commonly £150 to £400 for a typical domestic beam, though a long or heavy section costs more and may need a crane or genie lift to install.
Installation is where costs vary most. Temporary propping, cutting out the wall, lifting the beam, building padstones and making good typically runs from around £1,500 to £4,000 or more for a single beam, depending on access, the condition of the existing brickwork and whether the wall carries chimney breasts or floors that need extra support. Always get the calculations done first, because pricing a job before the beam size is known is guesswork.
The process is simple. An engineer visits or reviews your drawings, measures the spans and what sits above the proposed opening, then produces calculations and a sketch your builder can price and build from, and building control can approve. For most single beam jobs this takes a few days to a week.
Be wary of a builder offering to size the beam by eye. Even if the guess is strong enough, building control will still want calculations, and an oversized beam wastes money on steel and labour while an undersized one risks cracking, sagging floors or worse. A short engineering appointment at the start is the cheapest insurance the project will ever buy.
Experienced builders often guess close, but building control will still require calculations from a structural engineer before signing the work off. Getting the calcs done first protects you and usually avoids paying for more steel than you need.
For a typical single beam over a knock through or rear extension, most engineers can turn calculations around within about a week of the site visit. Complex schemes with multiple beams, posts and foundations take longer.
Genuinely non load bearing walls can usually be removed without a beam, but walls that look non structural often carry ceiling joists, roof struts or stiffen the building. Have an engineer confirm it before removing anything, as this check is quick and inexpensive.