Are architects plans needed?

[Toddlerteaplease]

My friend is planning a single storey extension to the back of her house. Some builders want Architects plans and others don't. So she's a bit confused.

Does it require planning permission or is it within allowable dimensions?

No it falls within permitted development. So no planning permission needed.

Some builders use architect's plans to price from - it also shows the client is serious about proceeding. Actually, structural engineer/building regs plans would be more helpful for the costing. Some builders will price without plans as they know the job/scope etc. She'll need plans anyway to apply for her certificate for PD; she'll also need building regs although she can do this on Notice, rather than Plans. Plus party wall agreements if she has close neighbours. Finally, she may not need an actual architect, an architectural designer may suffice. All planners are worried about are the elevations, rather than internal layouts.

At the least the builder will require structural calculations so shall need those drawings which should suffice.

an architectural technician can do the plans for a simple extension a lot cheaper