Hi, I have a shower room next to my bedroom and would like to add a WC and basin. The shower waste is connected by approximately 2.5 metres of 40mm pipe which goes through the wall to a boss connection at the top of the soil stack in the eaves of the roof with an AAV as shown in the photo. To add the WC and basin I understand that I will need to increase the height of the stack vent so that it is higher than the basin overflow but unfortunately there is no space above the AAV. Therefore I was planning to replace the boss connection with a 92.5 degree bend which will then run approximately 2 metres to the WC (in the same direction as the current shower waste). At 1 metre I was going to add a soil branch which will provide a vertical vent pipe around 1 metre long with the AAV at the top. The basin and shower waste will be teed together and fed in at the soil branch.
Does this approach comply with regulations? Is it sensible? Any advice/suggestions would be much appreciated!