Foul smell loft bathroom sink

[Placestogo]

We’ve had the loft done last january. The bathroom is brand new. So why is there a seeage smell everytime i use the bathroom sink? It seems like if water start pouring down the sink the smells come up.
for some reason, no issues when running the shower or using the toilets.
any advice appreciated thanks

You haven't got a macerator, have you?

An add-on bathroom, after the house is built, often has sub-optimal pipe runs. Perhaps there is a long horizontal run. Draw us a sketch of the route you think the soil pipe and waste pipes take. Look at the Building Regulations plans.

The basin waste shoild have a trap to prevent odours coming up the pipe. Show us a pic of the pipes underneath please.

Are you sure the odour is not coming up the shower waste?

The soil pipe should be ventilated, preferably by the top of the pipe venting into open air above the eaves and any opening windows. It might have a durgo, possibly hidden away behind panelling.

Sometimes a sewage smell is the first sign of the sewer outside starting to block up and ithe smell is venting at the highest opening in the house.