The answer to this depends on what it is you are doing to your house, but generally speaking you would need to consult a structural engineer on any project that involves removing load-bearing walls or chimney stacks, cutting timbers out of a roof structure, widening doorways or windows, digging basements or lowering floors, or building on soft or contaminated ground, or near large trees. Basically any job where you need a specialist to assess whether a new structure will be safe, or whether an alteration will affect the safety of the existing building. Your builder or architect should have a good idea whether you need to call a structural engineer - alternatively, while you're getting building regulations sorted out, ask the building control people if they think it's necessary.

A chartered engineer is a qualified member of a professional institution carrying a royal charter. It carries reassurance that the individual concerned has undertaken a rigorous qualification process in order to practice as an engineer. The word chartered can only apply to an individual, not a company.

An architect deals with the form and function of the building, and while s/he will be able to come up with ways of making the structural engineer's recommendations fit onto the plans, s/he doesn't have the engineering know-how to assess the safety of the building. Having a structural engineer's report on your project will give you peace of mind but should also help you when you come to sell your property and the buyer's solicitor starts asking difficult questions about alterations.

You're looking at between £450 and £800 per day (including VAT) for the services of a chartered engineer, depending on experience, regional variations and so on. For a relatively simple project, which would require a site visit and structural calculations being prepared, a day would probably be all that was needed. Once things get more complicated and you need further site visits or structural drawings, you may have to pay for a week or more of their time.