1. Their experience
Architects who have a lot of experience working on house extensions will be more likely to help you achieve your requirements on time and on budget.
Asking about an Architect's previous work is important when looking for the best architect for you. They should also be well versed in current building codes and regulations that may apply to your project. You can normally find this within the services section of their website or when speaking with them.
It can be a good idea to ask about previous projects, particularly projects which are similar to yours. This will give you an impression of the architects previous foresight and handling of a project and how that can apply to you when working with them.