Downspout into soil waste pipe?

You shouldn't connect a downpipe to a soilpipe because without a trap there's nothing to prevent foul odour from venting up the downpipe. The downpipe needs to be taken to the ground first then If theres nowhere to run the pipe you can create a soak away for it by digging down 600mm deep by 500mm wide and filling the hole with chippings then concrete on top. Alternatively you could dig a trench to fit a trap and run a pipe to a near manhole with a slight fall.

Hi there.

I dont think it is a question of legality, its more, would you want to. If you have a blockage of the waste soil pipe, its going to travel back up your guttering. The slightest leak at the bottom of your downpipe and you could end up with sewage all over your garden.

Hope this helps.

Would this downspout be coming from the roof guttering by chance ? It is possible to have a combined network. In which both storm and foul leaves your property on the same line. But this is not always the case. Factors of any sort of building extensions also come in to play. Your local water authority should be able to tell you if you have a combined or separate network from their records.