When you reach an agreement with a building contractor for works on your property, it’s deemed as a contract, even if it’s just verbal and there’s nothing in writing. For instance, you might engage someone to cut your hedge but you’re unlikely to put it in writing, even though you’ll have agreed the hedge in question, how much is to be taken off, when it will happen and how much you’ll pay them.
Employing someone to work on your home construction project is no different to this, in principle. Of course, the scope of works is likely to be more complex, take longer, could involve arranging material supply and payment might need to be in phases. Typically, you’ll approach trades (if needed) by phone, text or email, leading to a meeting on site and quite possibly an exchange of documents. You’ll define the work needed, when it will start, how long it will take to finish, any specific quality issues, how much it’s going to cost and when you are going to make payment.