How much value does planning add to property?

ReadingTim

If you're hoping for some sort of 'automatic price reduction formula' that will cause the vendor to immediately roll over and accept your demand for a price reduction on the basis that it's "reasonable" or "fair" then you're sorely mistaken. 

Obviously, you can ask for whatever price reduction you like, and use whatever justification you fancy, be it based in reality or not.  Or not even justify it at all.  Ultimately it's just a matter of negotiation, and the vendor can accept, reject, or counter-offer, with the likelihood of one response over another potentially based on how good the relationship has been up to this point.  

You should decide what you'd do if the offer was rejected - accept the original price, or walk away?  Or call their bluff, but risk them calling yours back... A threat's meaningless if you're not going to back it up. And are you actually going to do the work the place has planning for, or just see it as a way to chip the price?  

Finally, I'm not a planning expert, but would image that reaplying for permission for a scheme that was previously approved, isn't contentious and in an area where policy hasn't massively changed shouldn't be that onerous, or expensive, so can't imagine you'll be that much out of pocket, especially given what the overall cost of the project is likely to be.