Can anyone recommend a good home renovation specialist working in the Mount Hawthorn area?

I can highly recommend Amerex Renovations and Additions.

Definitely don't use Summit home improvements, as they are the shonkest ever! (speaking from experience) Such BAD quality & BAD service & follow up. We are still having problems now. Ended up not paying final balance due & they said they would come & fix all the issues to show that they were seriously sorry for all the stuff ups. Never heard from them again. Funny that! Now our windows have slid inside the wall cavity & there is a gap at the top & you can see the silver lining inside the wall cavity. We have called & emailed - but surprise, surprise, they haven't yet replied. (2 weeks so far) We are going to contact the W.A Building Commission & discuss it with them. It also took over a year to finish the extension & stuff ups when it they said it would be done in 5-6 months.
I'd recommend you try Bella Casa - we know 2 people who were very happy with their work & also communication. Jobs started & finished in the agreed time frame & pretty much on the agreed budget.
Also try Amerex - you may pay a bit more but they do top quality extensions & also Add Style - we had them do up a plan & give us a quote. The Summit plan was a better design but geez we regret not asking Add Style to redo their design. We would've been way better off using them.
Your home is your greatest asset & you want to know that any extension that you have done lasts. After all, its a lot of money that your spending & you should be able to expect the job to be done with a certain quality. Not just throw together.
Good luck! (: