Just had a builder round to look at closing off the side passage. This new room joins to the exisiting back door creating a small storage room.

The work needed:

1. Raise the floor to the house level- concrete slab 1.5m x 1.5m x 10cm

2. Build a wall and render (from house to boundary wall) 1.5m x 3m high.

3. Add a wooden door and frame to the wall. (door and frame price 3,000)

4. Extend the current cement sheet roof using 3 sheets to enclose the room- approx 3.5m2.

Total inclusive price 29,000baht

Is that reasonable?

Where in Thailand are you located?

I would say it's not unreasonable for Bangkok area but expensive if you are in darkest Isaan.

You should always ask for a couple of estimates of course!