I am about to start building a western style kitchen at the back of my house. The space available is 8 meters wide and 4,3 meters long on one side and 3,8 meters on the other side. There is already a 10 - 15 cm thick reinforced concrete slab covering the whole of this area. I will add another 20 cm of reinforced concrete to make it level with the rest of the house. I will use superblock as load-bearing walls. I have not made up my mind if I just put a roof on top of the walls, or I make a concrete foor on top and use it as a balcony. Maybe even make an extra 2nd floor bedroom in the future. I was wondering if the concrete slab would be sufficient to carry the load if I go with the last option. The ground undre the concrete is hard dry dirt and gravel. There is also no problem with water or flooding in my area.

Any input from people with knowledge in the field is appreciated

Cheers

That's a really thick concrete base, but I have seen the specs for q-con lightweight blocks used as load bearing walls and it is pretty rigorous and will pass some of their specs as a guideline. Footings under the wall perimeter are roughly 40x40cm and slab is minimum 10cm thick. There are charts for sizing the footings and rebar appropriately. 25cm thick blocks should be used for exterior load-bearing walls, not the small ones like the normal 7.5's. They sell floor panels you can lay across the blocks for making the second level. Since in your case you don't have the flexibility to follow the standards for a new build so you should consult a qualified engineer. Perhaps you can get by with less than an entire house would normally need. For sure make sure you get the proper rebar & concrete strength for that second slab.

A few more things for load bearing walls. You need a damp proof course slip joint between the blocks and the slab to allow for differential movement. You also need to properly secure the roof to the wall. There are a lot of details like this that most Thai builders will not know how to do and will be omitted so be sure all of the superblock specs are understood and followed precisely.

Sounds good to go and good for a 1st floor if or when you go ahead.

Just make sure you let the extra over floor slab cure fully before the walls go up.

Think not only of the floor strength ... but the compression loading on the footings (presuming it's not a floating slab).

You don't want the sinking feeling!