Buying land with an existing house on it
An existing house on a site can be the key to securing planning permission on what would otherwise be a greenfield site. But in some instances the planners will insist you keep the structure and include it in your plans. From a cost point of view this tends to be a more expensive option than knocking down and starting from scratch; there are of course conservation issues to consider.
Half completed houses aren’t as commonly available as they used to be in the past but may be worth considering; for example if the house has been damaged by the elements beyond the point where an engineer can guarantee its structural integrity, or where the insulation levels initially installed are not up to scratch, it can be easier to knock it down and start fresh. Builders also usually prefer to work with new builds rather than existing structures (for which they may factor in a risk premium into their price).