Anyone worked with Resi or other companies to plan and design side return extension

We’re finally beginning to plan a side return extension. We came across a firm called Resi and were attracted by their sort of one-stop-shop service to designing and planning extensions..

They have apparently done quite a few projects around the Ladder. Would anybody be willing to share their experience of working with Resi? Alternatively, has anyone worked with another design and build firm or architect for the planning etc of a side return extension.