We recently completed a building work in our house and we paid in full what was agreed (signed and some extras). Then, the builder came back saying that he made mistakes in his calculations and did things he never told us and we should pay for all this extra work. He never informed us or told us in advance. In fact, his work has been really poor and he took 15 months to complete something he promised to complete in 3 months. He only came on Saturday and made our life miserable for more than a year. He is now bullying me and threatening me to increase the amount I have to pay him. This situation is horrible and nobody seems to give me a clear advice about my legal rights. Can he demand what he pleases me and get away with it? Please, any advise will be much appreciated.