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looking at a RIBA approved architect as a start does give you more security as they are more accountable to undertake work. I honestly believe we think professionals cost and then you should see what an amateur can cost in the long run. You can also book (small cost) an initial planning advice meeting with the planning department which is a great way of talking them through what you would like to know what you can and cannot do.
I also would really explore whether bigger is better and are you better really upgrading the space rather than expanding. Cost per sqf is becoming more questionable in terms of extending. It can actually be more economical not to extend so much but really elevate materials, finishes, the bones, insulation, performance of the home as a whole. Something to provide some food for thought.