Thx for your feedback guys. After many hours of experimentation I’ve found 3 ways to do it using deep trench fill foundations.
Method 1: Select “Walls with footings” and “slab at top of stem wall’ in the foundations defaults.
Method 2: Do not select “slab at top of stem wall” in the foundation defaults but in the foundation wall structure select “Hang Floor Platform Above on Wall”” and “Subflooring to wall Interior”. Then select for the ‘room’ that is bounded by the foundations that “Room Supplies Floor for the Room Above” and specify the floor structure to be concrete. This allows me to place the concrete floor any between the top of the stem wall and the bottom of the footings.
Method 3: Use the same foundation defaults and foundation wall structure as method 2, but don’t select that the “foundation room” provides the floor for the room above. In the room above specify for the floor as being concrete and it will place it at the top of the stem wall.
The attached pictures show methods 2 and 3. Method 2 is the right hand side and method 3 is the left hand side. I’ve set the floors at different heights to distinguish the two approaches.
It’s still not perfect because I actually want to place the first floor so that it overlaps the top of the stem wall and the bottom of the exterior and interior walls. I’ve not found a way to do this as it appears to me the HD Pro defines wall heights of rooms by the position of the floor platforms that bound the rooms, and it doesn’t support floor placement that encroaches into the rooms that are above or below. I actually want to be able to specify the wall heights and the position of the floor platforms independently, or change the composition of a wall part way up. The other issue is that I want to specify more layers in the floor than the floor structures in HD Pro seem to allow. I’ll might put these out as separate posts to see if anyone has any ideas.
Re grade beams, or “pile and beam” as I know them; they only tend to be used in the UK when there are difficult ground conditions. The vast majority of houses are built using deep trench fill and it’s difficult to find local builders that know anything else. All sorts of new foundation systems are being introduced but the take up is slow. Some companies offer piled foundations with a reinforced concrete slab that is laid within a metal tray that is raised up from the piles so it’s suspended in the air, but such systems are expensive and the adoption rate is minuscule. Another type of foundation used for difficult ground is “reinforced raft” where all the first floor walls of a house are laid on a thick reinforced concrete slab that ‘floats’ on the ground. However, for ‘acceptable’ ground conditions, cost effectiveness, speed of build and an available labour force, deep trench fill is still the way to go.