An inspection of the land before purchase is an obvious necessity whereas the extent to which you might want to have a detailed professional survey carried out depends on several factors including:
Whether the owner will allow holes to be dug. A vendor will often allow a limited number of small trial holes to be dug so that you can get an idea of the type of foundations necessary. The cost of a JCB and driver for half a day along with a structural engineer for a couple of hours may be money well spent, especially if the engineer notes other potential problems such as trees that might be close to foundations, unstable or disturbed ground and water table levels. You will need to fill in the holes after inspection.
Any existing buildings you might intend to retain should be surveyed to establish their condition of repair.
If previous inquiries have revealed a possibility of archaeological remains then you should try to assess how this might affect building work. You should be able to get help from the Development Control Archaeologist in the local planning department.
If the site or any existing buildings are known to contain any protected species then try to assess how this might affect development. You can contact the local planning department to find out if the site is a SSSISite of Special Scientific Interest.
The site might have physical problems. For instance it might be very steeply sloping, it might have remains of old buildings and foundations, mining subsidence or it might have existing drains or watercourses running beneath it. If it is a brownfield site which has previously been used for industrial purposes (rather than domestic) then site remediationthe term applied to the method of dealing with pollutants and contaminants in the ground. The Building Regulations cover this in detail. See info about Approved Document C – Site preparation and resistance to contaminants and moisture may be insisted on by the building inspector. (see Resistance to Contaminants) This can be an expensive undertaking and if there is any indication of this problem it may be best to ask the vendor for permission to do a more detailed survey to establish the seriousness of the problem. An engineer can help with these matters.
Other environmental factors. Again the Environment Agency on line maps cover a variety of issues from Coastal Erosion through Groundwater source protection zones, Landfill, Pollution and Air Pollution,
Conditions that the vendor may want to include in covenants particularly if they intend to make you build boundary[for the purposes of part B of the Approved Documents] - The boundary of the land belonging to the building, or where the land abuts a road, railway, canal or river, the centre line of that road, railway, canal or river (See Diagram 17.)
[for the purposes of part J of the Approved Documents] the boudary is defined here walls or fences) may be affected by the nature of the plot.