We've got the same situation but don't regard it as a problem. We bought the house in 1991 and it needed decorating and minor updating.It was at least 5 years later that we got around to putting in a new kitchen and discovered the manhole in what was an extension to accommodate an enlarged kitchen and was built in 1975! We decided to leave well alone and that is still the situation now in 2009. The previous owner who added the extension was an architect by profession. Again we thought it strange how the plans were passed (we're in England)but it was a long time ago. We've never had any problems and don't lose any sleep over it. Relax!