We have done a few of these for clients and depending on where the property is located I can organise a quote for you.
The two main things to consider is in terms of how far you want to go with the conversion:
- do you want it to be just a storage space (not nicely plastered, no nice floor covering, no nice light fitting, still exposed beams, etc)
- do you want it to be finished like a proper room
Depending on how renovated you want it to be, is going to have a big impact on your renovation cost.
p.s. Make sure that the appropriate permits are obtained for your attick conversion.