If you do go down the auction route, it would be worth talking to the planners about what sort of permissions my be granted for change of use,
In the street and surrounding streets, what are similar properties being used for,
When we buy at auction, we often are getting development properties at a low price than if we were buying them on the open market,
Something like your property will get a higher price if developers know there is already permission to turn it into flats.
that trending in the area, since you bought the property, what has changed in the immediate area,
go on the local planning website, read all the permissions granted, and all the proposals for local properties.
then go on zoopla or similar, look at the prices all the other properties in the road area have gone for, then compare any that are similar to yours.
once you have that information, you will know how to adjust your proper to the target audience.
has your original planning permission lapsed ?