Did you get consent for storm to sewer easy enough? Our next place has clay ground. Last time I built near there they let me send storm into the combined due to ground conditions. Our site currently has a garage with an existing connection which I'm assuming is into combined.
I was chatting to a guy at a firm i did some work for the other week. I think these engineers err on the side of caution now they're not happy with 1 years cycle to restore the ground.
This thread is one to watch for me. Our new plot is similar. We have clay ground. There are 5 apple trees I've cut down. 2 on the building line or within it, 3 just outside. I'm pretty confident we're going to go with trench fill foundations. I've gone as deep as 2.2 previously. I will get a plasticity test done on some samples when I have the site cleared. My understanding is 2 holes. 1 near trees with 3 samples at 1m,2m and 3m. Then one away from the trees. Give these results to engineer to see if medium or high plasticity.