When you hire a construction company, they give you a quote and you sign the contract based on their projections. They will tell you what they think the project will cost and how long they expect it to take. You review the details before you sign.
Experts warn that it is often difficult for people who do not have training and experience to read lot lines and understand property boundaries. You may have a general sense of where your property ends, but do you know exactly what it stops and the next lot begins? A lot of people do not, so they hire professionals before buying homes or having them built.