Hi, yes the extension has full Building Control approval, there is a slot drain that runs around the perimeter of the sliding doors, also the terrace itself is not enclosed and falls away from the house, so surface water cannot build up and discharges into the garden.

Thank you for this, we must have a particularly difficult building inspector then as he told us he would not accept a slimline drain channel even though our space is also not enclosed and a large lawn area is within a couple of meters from the doors... baffled!

Yes that does seem odd, it is a pretty standard detail.

The slot drain is only there is prevent wind driven rain being driven under the doors, although the sliding door tracks also present a barrier and have drain holes in them that discharge into the side of the slot drain below ground. I would ask him what his concerns are and maybe consider going over his head....