Redesign of loft conversion

[SternlyVoice]

Our house has a loft conversion but it is poorly designed and doesn't make good use of the space. We'd like to change it from having a large landing, one bedroom and a large shower room into a conversion with a second bedroom, a smaller family bathroom and an ensuite added to the existing bedroom. We'd also like to add a couple of dormer windows and are willing to extend into some additional roofspace, currently not converted (would involve raising roof level to match rest of house), to get a decent sized second bedroom.

Should we just approach loft conversion companies for a one-stop shop solution or is it worth getting an independent architect to first draw up some potential plans and then approach local builders for quotes? What's the best approach or what lessons did you learn from your loft conversion?

Comments very welcome!