That's a very good question
In my opinion you do need to use a licensed builder. By using a licensed builder what it does is ensures everything is done correctly. Unless you have the means and the knowledge and the tools to be able to facilitate yourself.
You see where not talking about a letter box here, where talking about adding a permanent extension to more than likely your most important asset, your home.
We get several phone calls a week of failed DIYs that they are asking us to help salvage however the reality is by that stage its to late and we can't help you.
Like the old saying in life if you try and cut corners most likely it will cost you more in the long run. By using a licensed builder you ensure all legal aspects are taken care of, by the way of permits, pluming and engineering and more importantly the area you have built is safe to sit under cos lets face it it is there for your friends and family, you want to make sure its safe for all people to be underneath.
The other reason why its good to use a licensed builder is you have the peace of mind of all the warranties and in my experience the larger the warranty the company offers the more confident they are in what they are doing.