I extended and converted a three bed semi into 2No.one bed and 2No.two bed flats during 2004-2005.From gaining permission to completion and tenanted took just over 1year but there was a bit of a hold up with planning,so one year would be a realistic time scale for a similar project.
A neighbour is an amateur film buff and recorded this short film of work in progress.
Visit: www.hemelmoviemakers.org.uk (go to films then click Bill's Project)
Before starting the works, I visited Building Control for advice on the then relatively recent requirement for soundproofing and the inspector suggested a company who manufactured a floor system for this type of construction which I did.A design was approved by B.C. and I followed the specification to the letter because by this time I was well aware of the severity of sound testing and the cost which was £1000 each time for the four flats.
The works went very smoothly,perhaps because I was project managing but more likely I have the respect of great tradesmen who I can turn to.You must be confident of anyone's ability who you employ as well as your own, as mistakes can be very costly.
So when it came to testing at the last knockings I was reasonably confident of a pass but when the recordings were analysed I received good and bad news.The original side of the property passed but the new build failed, simply because B.C. had approved the specification for the internal leaf blocks for the building as adequate when in fact they weren't dense enough.The Impact Test passed on both the new and the old but the Airborne Test failed on the new side.Basically,an unacceptable transmission passed through the block work,down the cavity and into the flat below,even though the cavity was 3" wide and full of Dritherm.
We had even dot/dabbed the face of the block work with 15m.m. "Soundblock" plasterboard prior to skimming and had properly isolated the boards from the floor!!
When I approached the floor system specialist they didn't want to know as their part had passed and I didn't want to fall out with B.C. as I needed the job to be signed off.Remedial works were carried out and after a re-test it passed (just).
I would certainly take on this type of project again as it's probably one of the most lucrative ways of adding value to property but it's essential you work out reasonably accurate costings(there should be enough of a buffer to cover contingencies and still come out with a very healthy profit).I actually went £40K over my original estimate but I hadn't allowed for fencing and landscaping which was a planning condition.
Most importantly, you engage an architect familiar with this type of conversion as well as qualified tradesmen to carry out the works,not to mention your honesty with you own capabilities.
These flats now form part of my rented portfolio which is mortgaged under one title.An owner of property wouldn't issue a lease to himself.
Hope the above is of interest.............Bill