Short answer: if your project changes anything that holds the building up, or you're worried something structural is wrong, yes. Here's the practical breakdown we give people who phone and ask exactly this.
You need an engineer if…
- You're removing or cutting into a load-bearing wall (and if you're not sure whether it's load-bearing - that's also an engineer question)
- You're building an extension - foundations, beams and roof all need designed
- You're converting a loft - new floor structure, altered rafters, steelwork
- Your builder or Building Control has asked for calculations
- There's cracking, movement, sloping floors or bulging walls you want assessed
- A survey during a house purchase flagged a structural issue
- You're building over or near a public sewer, or need a retaining wall
You probably don't need one for…
- Like-for-like replacements: a new kitchen in the same layout, replastering, rewiring
- Hairline cracking in new plaster or around new works - normal drying and settlement (though if it grows, ask)
- Non-load-bearing stud wall changes - though verifying a wall really is non-load-bearing is sometimes the whole job
- Fences, decking and garden structures of ordinary size
The cheap insurance principle
The cost of an engineer on a domestic job is small against the cost of getting structure wrong - an undersized beam, a foundation that needed to be deeper, a wall that turned out to be doing more than anyone thought. Most of the remedial design work we do exists because someone skipped this step.