From experiences of friends, builders love working without a full contract as it means you can legally hold them to very little. Basically, they work to the plans and nothing more. It means they can put one socket in a room and say "that's what was quoted" and you have no argument to the opposite.
We had three builders refuse to quote as our tender was "too detailed". Said it all really.
Our contract specified:
Responsibilities of the builder - for eg legal responsibility to get rid of waste in compliance with the law
Anticipated timelines
Payment schedule and grounds to withhold at the discretion of the client
Cancellation options. Ie the builder isn't turning up and project is falling behind....this one really upset him as he said he might have other jobs on......this meant he was on site virtually every day weather permitting
Referenced the latest approved plans and building control plans
All detail not included in the plans, for eg the exact type of ceiling windows, ie Velux and model number and not some cheap alternative
Window design, make and model
Door design, make and model including colour
All electrics, including cables for cable TV, Internet IP6
Plumbing...including make, model and importantly boiler capacity, type of piping, for example copper 15mm or small bore
This is still a small part of the overall list which should match the tender the builder worked to.
You cannot start a major project such as this without the level of detail given the cost, Also the Architect won't help if this spec isn't in place as the builder will be complying with the requirements which are very few!
Sadly, you can see I have learnt a lot about detail but what I can say is the building work was fabulous and done to a high standard. The builder admitted initially he didn't like the contract provisions but that what was useful was that it was so specific there was little area of doubt with anything and he knew to discuss any changes with us and get authorisation....in writing or email
Good luck but it feels like you have a lot of work to do before letting him start....don't let him push you