I know that I'm not allowed to wash my _car_ at home, in my garden, garage, in front of my garage, on the street etc. (unless I have the proper sewage / waste system installed).
I don't know about washing a bike however. I understand that bikes have oily parts (eg. the chain) which can still be harmful to the environment. I would argue however that the oil coming off my bike during a wash is so miniscule that it does not matter. I do ride mountain bikes, and I would like to get rid of the dust / mud after some rides. I also use bio-degradable cleaning products.
What does the law say? Does it say anything about washing bicycles at home?
No thanks, that's only as a last reserve (and it would leave the bath unusually dirty).
I can take the bike to a nearby car wash, but it costs extra money (and the wash accepts coins only which I don't always have with me), and extra time.
The amount of tyre residue washed into the drainage system off the roads from all the giant BMW and Mercedes SUVs with their massive tyres makes a few soap suds from washing a bicycle seem a bit trivial.
My bike mechanic spouse recommends for mountain bike cleaning to allow the mud to dry completely then remove as much as you can with brushes and finish off with a bit of bio bike cleaner on a rag. He never uses a hose to clean bikes as water can get into all sorts of places where it can cause problems.
This is the "best practice". However, sometimes the bike and me are soooo covered up in mud that something must be done.
If you're in a bike park, ask for the cleaning area. There's a low pressure water pump which doesn't harm the bike by pushing water through the rubber seals aimed at keeping it out.
2nd best, go to a car wash and keep the nozzle as far away as possible from the bike (80-100 cm). I end up washing my shoes some times too, otherwise I'm denied access to my apartment
Somehow I have the same problem now, but I slowly got used to riding a "dirty" bike. I haven't clean my bike since last year just riding it as it is, excess mud falls off on its own Luckily I have a bike garage in front of my building so I don't have to care about leaving dirt in the cellar area.