Dim is it? Is there another dim lamp somewhere? Have you still got old-school proper tungsten light bulbs (low voltage or 240v). It is highly possible that your builders' 'electrician' has cocked up in such a way that one lamp is in series with another lamp with a certain combination of switch positions. Again, annoying rather than dangerous as long as the cabling is protected by a correctly rated circuit breaker/fuse and as long as your builder gets the 'electrician' back to sort it in a timely fashion (like 24 hours maximum).