Stephen Turvil [www.stephenturvilarchitects.com
] - we're using him right now for a side return project. He's been great. We used an architect because our building has had various add-ons built over the years (not by us) and we have a dropped ceiling above the kitchen, and altogether our side return is not at all straight forward. Stephen has had some great ideas and we have arrived at a plan we're very excited about. If yours is a straight forward job, I'd just use the architect your builder uses for drawings for planning permission etc.
Side return extensions are usually an expensive undertaking, I think, as they involve new foundations and a lot of 'steel'. We're expecting ours to cost twice what it cost us to convert the loft, and perhaps a bit more, for example. Having said that, ours involves fixing the first floor issues as well.