So, yes, you will be able to let an attached self-contained unit under the rent-a-room scheme as it satisfies Revenue's criteria.
Never assume with planning, an agent selling a property will tell you what they think you want to hear. You need to find out what conditions were made on the grant of planning. Your local authority will be able to give you that, or you may even find the planning details online. Your solicitor should be confirming all planning is in order before you proceed.
Ultimately, unless your LA are really on the ball, or someone reports you, they're unlikely to find out. If they do though, they can come after you with an enforcement order.