billythekid
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from approved doc H
2.2 Some public sewers may carry foul water and rainwater in the same pipes. If the drainage system is also to carry rainwater to such a sewer, the following provisions still apply but the pipe sizes may need to be increased to carry the combined flows (see paragraph 2.35). In some circumstances, separate drainage should still be provided (see Approved Document H5).
and
2.8 Combined and rainwater sewers are designed to surcharge (i.e. the water level in the manhole rises above the top of the pipe) in heavy rainfall. Some foul sewers also receive rainwater and therefore surcharge. For lowlying sites (where the ground level of the site or the level of a basement is below the ground level at the point where the drainage connects to the public sewer) care should be taken to ensure that the property is not at increased risk of flooding. In all such cases the sewerage undertaker should be consulted to determine the extent and possible frequency of the likely surcharge. 2.9 For basements containing sanitary appliances, where the risk of flooding due to surcharge of the sewer is considered by the sewerage undertaker to be high, the drainage from the basement should be pumped (see paragraphs 2.36 to 2.39). Where the risk is considered to be low an anti-flooding valve should be installed on the drainage from the basement. 2.10 For other low lying sites (i.e. not basements) where risk is considered low, sufficient protection for the building may be possible by provision of a gully outside the building at least 75mm below the floor level. This should be positioned so that any flooding from the gully will not damage any buildings. In higher risk areas an anti-flooding valve should be provided, or the drainage system pumped (see paragraph 2.36 to 2.39). 2.11 Anti-flooding valves should preferably be of the double valve type, and should be suitable for foul water and have a manual closure device. They should comply with the requirements of prEN 13564. A single valve should not normally serve more than one building. A notice should be provided inside the building to indicate that the system is drained through such a valve. This notice should also indicate the location of any manual override, and include advice on necessary maintenance.
So the answer is yeah its kosher, if yer drains can handle it. In fact most houses here are combined rainwater+grey water. Not WCs though.
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