sample contracts are downloadable - google it
RIAI or CIF or similar
might be enough to have a scope of works with a fixed price on a single page and signed by both parties depends on scale of project
there is no reason you cannot draw up own contract as long as it doesnt breach law of the land, whether the other party signs it is another thing.
basic clauses re start, finish, costs and payments, quality, health and safety, variations (changes), etc etc are needed
always better to have something on paper pref with a signature!
remember contract works both ways can be used against client too
in reality for small building jobs breach of contract that is pursued by one of the parties will end up in small claims court and is worthwhile having some evidence of an agreement