I agree that you would appear to be correct in your surmise that this work is liable to VAT at the reduced rate of 5%. The regulation reads:
'Qualifying residential premises’ means:
You need to issue a certificate to your contractor to the effect that the reduced rate of VAT applies [see paragraph 1.3 of the VAT notice 708].
In reality it does not matter save as regards ***** ***** as, if your contractor's invoices are at 20% VAT, it is simply reclaimable input tax whilst you apply the correct output tax at the reduced rate on your invoices to your client at 5% and recover through your quarterly VAT return. As my old boss was wont to say 'It doesn't alter the price of cheese!'
I do hope my reply has been of assistance.