Do I need a registered builder?

Hi,
I want to extend my house and cannot decide whether to go owner builder or use a registered builder. Of course the quotes from registered builders are a lot more than from the carpenter I had do previous work. Looking at the owner builder paperwork it says I need to use registered tradespeople, how do you get around this? as I am sure extensions are sometimes done by unregistered tradesmen.
The builders quotes I have had have also added 15% for overheads is this normal?
Thanks
S


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