House Extension - What to put into the contract?

I suggest you are jumping the gun and going about this all in an unusual way.

You need Planning permission with all attached clauses met. You then need Buildings Regulations with all clauses met. This will be via a Full Plans Application so you have detailed drawings - to an extent. You may need working drawings, but you certainly need a detailed specification. To this you might add a full kitchen design, a heating design, similarly electrics, and roof details. Some may be cursory - I have no idea how extensive your project is.

Once you have all this then you can approach builders. You should not be approaching them yet because you have far too many loose ends to tie up first.

A Contract is worthwhile as much as anything to show the builders that you have recognised your input and position, and likewise respect their position as professionals.

But what I am telling you should all be coming from your Architect. It is all fundamental, simple, common sense building. This prompts the question why is your architect not inputting into all this and providing you with answers?

we have just been down this route, you really need to be on your game as building control are pretty useless in reality. now th ebuilder has left site with the job incomplete we are finding lots of cut corners and things he has done wrong despite having references, a payment plan, building control round to check at the agreed stages etc. To put the roof to the spec he was given and agreed to is going to be around £9000, to make the underfloor heating meet regs another £2500, there is insulation missing which means completey removing the kitchen ceiling and replastering and repainting it, he fitted our kitchen and has used silicone as glue to stick panels on and also put screws through the base of our £15000 kitchen units. \If you can pay the architect to manage the project and do checks its not a bad idea despite the extra cost. building control cant be there to check everything and unless you are a builder its very difficult for you to do it, we needed a new water main running from the water meter to the house and he told us it would mean a 12" trench cutting and digging through our drive which he would backfill, its 3 ft wide and he has backfilled it with sand! our old but good crazy paving driveway is now going to cost £5000 to put right.
I'm not suggesting you have found a dodgy builder but we dotted all the i's and crossed the t's and still ended up wiith a nightmare, the 3 month job rolled into 7 months and now we have to persue a lengthy and costly court case to get the £20,000 or so for the remedial repairs needed.
Our job was around £60,000 and it would have been less stressfull, quicker and better all round if we had paid someone else £5-10k or whatever the cost was to manage and take the responsibility for it all, unless you are knowledgable about all aspects of building work i would consider getting someone to manage it, our builder was local, reccomended, we visited 2 jobs he had done and spoke to the people that lived there etc, written quote, his home address, payment and works schedule , all money was signed for etc.

A contract is no guarantee that the work will be done properly.