I notice that there are a few redundant water tanks for sale on this auction site? Just wondering if they might make good/cheap hut sites? They will all have vehicular access of some kind & usually a fence & gate. Maybe planners would approve a hut more easily than a dwelling house? Good use of sites that are not attractive for any other kind of development.
http://www.sva-auctions.co.uk/html/Lots/February2020/lot1_details.php
Permahectic is saying the same as me, not all access means vehicle access or right to develop.
You need to check small or seek legal advice.