Your building contractor will discuss the work with you and give you an estimate of the cost. If you have proper plans and specifications prepared, it’s always a good idea to ask for quotations from more than one reputable building contractor. If it’s a small job that doesn’t need proper plans, it’s essential to provide your builder with a detailed written description of what you want and/or to explain to him your requirements carefully on site, so that he can ask questions. You can then ask him for a detailed estimate in writing. Large contracts can be put out to tender and where work is carried out under the supervision of an Architect, it’s wise to be guided by him.
Don’t forget, there’s a big difference between a cost estimate and a detailed, written offer to carry out the work. Don’t sign anything until you are sure that your builder’s offer is firm and clear and covers all your requirements