Small extension- where to start?

We are planning a small extension on the back of the house - usual thing of bringing out the back wall of the dining room to meet back wall of kitchen (possibly moving even further back with both walls by about 2 ft or so) and knocking the two walls through to make one big kitchen/living area. Where do we start? Do we talk to a builder first or an architect or do we even need an architect with such small job (the house is a relatively small 4bed detached).

Probably no need for an architect, most builder will draw out basic plans.

Watch out that supporting walls are not being knocked! reputable builder should help there.