Hi I am currently extending my house off the back of my kitchen where the kitchen sink currently drains into an outside gulley. The new extension will be built over the gulley and I want to extend this and attach it to a sink and dishwasher on a kitchen island. It will be an internal drain which will need to be airtight as I don't want foul smells from my neighbours toilet coming into the house. Will a u bend from the sink be sufficient to stop foul smells? I'm panicking about what to do at the minute.