A kitchen extension may involve a new roof, an opening through the existing building and connections between old and new construction. There is no default RSJ size that resolves that combination. Earlier generic capacities and project-price claims on this page have been removed.
Prepare a coordinated brief
Show the desired opening, roof arrangement, glazing, retained walls and any column positions. Include ceiling-height expectations, cabinetry constraints and services. A flush finish can affect how the structural arrangement is detailed; it should be discussed before fabrication.
Resolve supports and connections
Ask the engineer to identify the complete load path and any investigation needed at existing supports. Confirm how drawings for the extension and alteration fit together. Changes to glazing or roof layout should be checked for structural consequences before the steel order is finalized.
Agree the construction scope
Identify temporary works, access, unloading, inspections, fire protection and making good. HSE structural-stability guidance covers the alteration context. Confirm planning and Building Regulations requirements for the whole proposal, not just the beam.
Obtain dated quotes against the issued design and use the budget calculator to total them. The calculator cannot infer an extension’s costs or beam specification from the room dimensions.
Structural work requires a chartered structural engineer and the applicable Building Control approval. This educational content has not received an independent engineering review.