OnlyAsking...
I would suggest that you look at things in this order.
Engage a Chartered Architect or Chartered Building Surveyor (I may be available), he/she will be able to advise you on how best to proceed. But generally it ought to be something like this.
Overall quick appraisal, is it feasible to do what you want? Are you likely to get planning permission? Are there any matters such as the Party Wall Etc Act, or Easements such as rights to light that you need to consider? Are there any covenants in your deeds that may be relevant?
Having looked at those, and possibly other, matters, the next steps would be:
Planning permission.......even if it is permitted development I would still advise that you apply for a certificate of lawful development. I would never advise anyone to spend thousands of pounds without having first been given legal assurance that what they intend to do does not require planning permission.
Building Regulations consent...I normally advise residential clients of mine to go for a full plans submission. It helps to have the consent before you tender or negotiate a price with a builder as there is less likelyhood of any expensive changes having to be made.
Procuring the works.....What form of contract should you use? Do you want to competitively tender the works or negotiate a price with single builder? How long should the works take and what, if any, level of liquidated damages should be set for late or non completion? How will any disputes between you and the builder be dealt with? ......
Managing the works...how will the builder be paid? who will carry out any valuations? if the builders fails to perform what sanctions will you have and under what circumstances will you be able to dismiss the builder? Who will decide whether the quality of the works is adequate?
Of course you can try to do it all yourself, but frankly you have a much better chance of a happy ending if you get professional advice. If you must go it alone then at least make sure you have a proper contract in place. The Joint Contracts Tribunal publish a range of standard contracts that everyone in the industry is familiar with, including some that do not require the use of a consultant. See
here which gives guidance as to contract selection.
