Hi Pound,
I've done quite a lot of renovation work and indeed conversion of houses/bedsits into flats.
Set aside at least 5k for structural work and sound proofing.
£1000 for skips and clearance.
Re-wiring each flat will cost around £1500 each.
Central heating and running you hot and cold pipings will be about £2500 per flat tops.
Drainage and plumbing, soil pipes etc £1500
Depends if your walls all need re-plastering, I reckon about £1500 per flat.
New Bathroom with tiles, materials about £500, labour around £1500, so that would be about 4K.
Kitchens at least £3k per flat.
Joinery, skirting boards, doors, architrives etc about £1000 per flat.
Decoration around £1000 per flat.
Carpets, lino, underlay about £1000 per flat.
In total I reckon around £35 to £40 K.