One of the things to consider when having a home extension built is the relationship between the builder and the architect. If there is an issue on the build that the architect has missed or the builder does not carry out the architects plan there is room for problems. So we would always advise that you have a single point of responsibility, a company that are confident enough with their work and skills to take care of the whole design and build process. Then if there is an issue during the build phase of the project they have to take some responsibility for the corrective action.