Poor site drainage can lead to many problems in and around your home.
When inspecting a site, a walk over the entire area will help to identify any boggy or obviously wet areas.
Run off from neighbouring properties or the surfacing of natural ground water can accumulate in pockets and be held, even on sloping sites and especially in clay based or impermeable soils.
Water ponding under the house together with poor subfloor ventillation can lead to mould and rot attacking subfloor timbers.
Water ponding against foundations, especially in areas with clay soil profiles, can lead to cracking in brickwork and even to serious structural failures.
It should be noted that in most cases it will be the builders responsibility to install adequate drainage on the site to protect the home but it is the owners responsibility to maintain this drainage and to provide and maintain adequate drainage to the remainder of the site. In the event of a footing failure being caused by a lack of maintenance to the drainage system then the owner would be unlikely to have any recourse to the Builder or to any warranties.
Cut-off drains for surface water or water close to the surface are relatively inexpensive to install even if they require placement across the entire site.
(In many cases on sloping sites where cut and fill is carried out for the actual house site, a cut-off drain will need to be installed at the base of the cut as part of the building permit.)
The presence of water or damp areas does not mean that the site should be automatically discounted, but the possible need for control measures should be taken into account and budgeted for.
There could be the opportunity to purchase land that may at the outset seem to be overly wet and unsuitable to build upon, but by the placement of the appropriate drainage lines it may be possible to convert the site into a good piece of land.
Under building regulations it is a requirement that any roof run off (storm water) must be directed to a drainage system (legal point of discharge) and not flow onto adjoining properties. Wet areas on a site from natural run off may need to be addressed but illegal run off from an adjoining property may be your neighbours responsibility.
If in doubt seek the advise of a qualified consultant.