Legal bits that no one has mentioned.
This work is notifiable to the Local Authority.
If the electrical work is included in the extension building work then the building inspector probably won't deem you competent to do the work.
If not included, then either -
a) you must notify the LA before you start (hefty fee), which might have the same result regarding competence,
OR
b) you must employ a 'registered electrician' now who can supervise whatever work he says you may do.
This work is notifiable to the Local Authority.
If the electrical work is included in the extension building work then the building inspector probably won't deem you competent to do the work.
If not included, then either -
a) you must notify the LA before you start (hefty fee), which might have the same result regarding competence,
OR
b) you must employ a 'registered electrician' now who can supervise whatever work he says you may do.