1960's semi detached drainage plan

[AndyJay2019]

Hello all
Myself and my Wife are doing a DIY garden landscape which started during the pendemic (what else could you do?
We are now working on the patio area right outside the kitchen and have dug down approx 10" so we can lay the hard core and lean mix bed for the porcelain tiles.
We had a new kitchen a few years ago and the builders moved our kitchen sink and drainage to our extension (like an L shape now).
We noticed that the new (5 years) drainage overflows when the washing machine is on. Never noticed this before.
The old kitchen sink drain is still in place outside and nothing is flowing into this. There is also a flat roof (from the extension) rain water drain that drains into another drain. It looks like this drains into the same system.
This drain does have a bit of a stink to to so I tried to flush it through with a hose pipe. The colour of the water in the drain is a grey colour (bit odd).
Anyway, I'll get to the point...
I'm looking for a typical plan of a 1960's semi detached house so I can at least get some idea of what I'm looking at in out garden and how a typical drainage system works.
I have no idea. Where does all the water drain to?
I think I might take some photos and even draw a diragram to help explain what we have in place and hopefully someone can advise.
Thank you in advance and I'll post back with a better way of describing our situation.

Take Care
Andy

 

[Hugh Jaleak]

There is no such thing as a 'typical' drainage system, it would have been down to whatever would have worked at the time. Older properties often have what is known as a 'Combined' system, with rainwater and sewage going into the same drain, nowadays, they have to be separate, (hence known as a Separate' System, whereby the foul (sewage) drain goes off to be treated, and the rainwater can be put into the nearest suitable watercourse.

I can only suggest you lift any Inspection Chamber covers and try and gauge what is connected where, but without being on site, it would be virtually impossible to say what runs where.