The value of lifting drain covers for Homebuyer Surveys and Building Surveys cannot be underestimated. Often there is no sign of problems above ground and sometimes little evidence of problems below ground. Also where a property has single occupancy what was not much of an issue can, once a family move in with increased usage, become a far more significant problem.
Age of property can also be a problem as some drains during the 1960’s especially were built using pitch fibre drains which are quite brittle and easily crushed.
Tree roots can spread quite some distance laterally and just because there is no actual damage to a building does not mean that the drains are not defective.
I recently saw a bungalow that was affected by all these issues with the result that the drainage system was blocked/backing up in five of the six inspection chambers. The old lady who lived there probably had no idea as the only evidence was underground and with low usage did not overflow the inspection chambers.
Occasionally in reports I will recommend having a full drains survey and where this is stated you should always arrange for this to be done as drainage problems can run into thousand of pounds of repairs.