Drainage design for side and rear extension

[mak80]

Hey all,

After some advice on a drainage layout for a side and rear extension I have in the pipeline. Architect doesn’t do drainage so options are a diy or a civil engineer quoting me a tidy sum! If I have to resort to that I will but no harm in reaching out to the knowledgable people on here.

Property is a detached chalet bungalow so no upstairs for drainage purposes required. Drains are all private confirmed on cctv. Combined sewers in use. Extension is to the left side and to the rear of the original house. Garden extends to 35m.

I’ve attached a couple of images of ideas…maybe one is better than the other or maybe neither is workable at all. I haven’t put the pipe run for the family bathroom in the middle as not sure what the best way is for this one (the wc run really). 

For surface water, the underlying soil is clay so maybe a soak away is not the best idea…neighbour just did a (smaller) side extension and they just piped to existing gulley. But happy to get the drawing in with that shown then when it comes to it speak with BC on it.
Grateful for any advice!