Bath and sink waste into soil stack

Hi all, new here.
I’m having the back of my house rendered and want to tidy up the waste pipe work before and after.
Currently both 1st floor sink and bathroom waste pipes go into a cast iron hopper and from there down to the gully in 2 1/2” caste iron? This is close enough to the new (ish) plastic 110mm soil stack. Can I run the sink and bath wastes to the soils stack and attach with strap bosses or use the existing in line boss (same level as the loo waste) ? I could then do away with the hopper and down pipe. Hope this complies with current regs. Cheers. Lozza.

That’s fine, I’d change the pipes to black as well, so it all matches

Good plan, Hoppers are unsanitary things for waste pipes to go into. You wont get the lower waste into the boss on the junction, it's too high. Doesn't take much to fit a strap boss lower down, and then either common both pipes in before joining the stack, or run the basin waste to a suitable point higher up.

I was thinking of taking the sink into the existing boss and the bath to a new one. Not sure it’s a good idea to tee them into one and then into a new boss. Bath waste could cause some syphoning of the Sink waste.
The boss is angled up at 2.5 degrees I think, so I guess waste entry has to be fairly shallow, and an elbow is required in the waste pipes to shallow the fall.

If need be 'dog leg' the last section into the stack with a couple of 45° bends, gives you some wiggle room to get everything lined up.