I've worked for Build Team for over 8 years now and every step of my journey to becoming an Architect during those 8 years where supported and only possible through working at BuildTeam. They have helped me develop my architectural and leaderships skill. It has been a great place to work at and would recommend to anyone looking to learn how to design an extension, that you will actually get to see get built.
Cons
You will get given responsibility's that have a real impact on the business, which can be daunting at first but they work as a team, so that you never really feel alone in dealing with those responsibilities.