4 meter at the back?  Its so spacious ~~~~ just be aware of the set back requirement for an extension by the authority.

First and foremost, set an ideal budget. And how much for wet work and how much for ID.

Then you should have a clearer picture on where and how you wanna spend your money $$$, especially the economy is not good right now.

1. If your budget is very limited (<150k) > just change the internal partition and configuration. directly engage contractor is not a bad choice. just choose wisely. an experienced contractor can advise you on it as they are the one who does the work. They are more cost-sensitive. Designer and architect can be lost touch on the cost sometimes.

2. If your budget is just nice (>200k)> get an ID and figure out proper space planning with them. The extension plan can just go for the local council standard plan, so you can save money without the need of hiring an architect. Your end product will be more complete where ID have figure out all the furniture and lighting placement for you.

3. If your budget is unlimited > get an all-in-one architect who can take on the whole job for you. So you can just sit back and relax. Architect arranges the c&s engineer, settle the ID for you, appoint a contractor and even help you coordinate/monitor the construction work.

By the way, I am an ID contractor too.