I hope I'm ok to jump in on this thread, if not I'll start another.
We are starting off the process of doing similar gable end extension, albeit ours will be bigger, 75 sqm, 2 storey. We have a pretty good idea of the design we want to implement, we have architect drawings for same from some time back. We have always struggled with the idea of an architects fee of 10% of build cost - could we just engage with Engineer to do the full drawings and manage the PP process, oversee the build and sign off? My preference would be to use a QS to do the project management and tender docs side of things. Would that work?