Many properties in London have a shared sewerage system that is generally owned and maintained by Thames Water. There are strict guidelines on both working on and building on or adjacent to these sewers and it is necessary to gain approval from Thames Water to do so. Permission to place a new structure over or near a Public sewer is referred to as a Build Over Agreement. Applications are considered on a case by case basis and in some circumstances Thames Water will not grant permission place structures over their sewers. Where permission is granted, the work will usually need to be inspected by Thames water.
Search fees for existing sewers cost around £60 and generally take 5 working days.
Application fees for build over agreements start from £350 for a residential building over a standard size150mm diameter sewer. The application and consultation process usually takes upward of 4 weeks. Again, visiting the link below will provide a more detailed overview. http://www.thameswater.co.uk/cps/rde/xchg/prod/hs.xsl/693.htm