Your best bet here is to approach the council informally. Check on your council’s website – some councils have duty planning officers that you can call in and speak to, others ask you to write in and a few charge for such pre-application advice. You don’t need the owner’s permission to do this and any discussion with the planners will be in confidence. However, the only certain way to find out if planning permission is likely to be forthcoming is by making a formal application. Technically, there’s nothing to stop you doing this as long as you formally notify the owners. However, they will be less than impressed if you’ve made an application behind their back. An alternative would be to approach the owners and say you’d like to buy the plot, subject to getting planning permission. If they agree, you can get informal advice from the council.