If you're still considering that option, you should go into the planning office and have a look at the planning register. Thus is usually a map or a list with the planning history of an area or a site. Any member of the public can look up the planning history or a site or of a general area and this is exactly what you should do.
You can find out if there has been a planning application to built on that particular site before and you can also find out if the planners have been granting permission for building on sites in the area. That should give you a good idea of any relevant issues. The administrative staff in the planning offices are there to help you out.
You could also have a look at the development plan for the area, again this is accessible for viewing to all at council offices/internet. You should look for and designations affecting your site on the maps that are ancillary to the development plan. SPAs, SCAs, designations of special amenity, views of importance etc. could hinder your changes.
Also technically you are entitled to meet with the planners directly to discuss whether you would be granted planning permission as long as you have the permission of the landowner. This is common practice among developers. A letter from the landowner should suffice.
Your case for planning permission would be significantly streghtened if you work locally. Good luck!