I have appended what I think is an excellent link which discusses all items which are relevant to your proposed property extension. The following is a quote which is in the article concerned;-
//For most single storey extensions an architect is not necessary. Most experienced builders will be happy to work without one. If, however, you have unusual plans or very specific needs, it can be a good idea to use the services of an architect. //
In my opinion, if there is a good building firm in your area, that is a starting point for discussing matters. Personally, if possible, I would refrain from architects, which could add a high percentage of costs to your proposed extension.
https://www.myjobquote.co.uk/costs/single-story-extension
It would be interesting to hear what Answerbank's member 'The Builder' has to say on the matter; if by chance he sees your posting.
Best Wishes......Hans.