A plan is always good to have, so the builder can give an accurate quote for the works. The plan and specification allows the builder to understand what is involved and how long it will take to complete the project.
The client can also fall back on the plan if the builder has not performed to what is expected.
So the answer to your question is to appoint an Architect first.

It's entirely up to yourself you can get a builder round to have a look most builders will know the general cost of steels etc so they can put that in your quote if it's all straight forward that is.... and the same with your extension so I'd say get the builder round and get you a ball park figure, the architect will only draw up your plans and basically can tell you what you can and can't do.

Traditionally you will provide plans drawn by a designer first but It is often a good plan to instruct a builder and architectural designer at the same time as they can work together to provide a most cost effective solution