when i open that 'floor' the add walls etc is greyed out.
When you open another floor, all the elements inserted on other floors are greyed out. As soon as you draw something on this floor it will be in the normal colour. If everthing on other floors was in full colour, it would be almost impossible to work with, especially if you had many storeys, it would be very confusing trying to work out which floor was which.
On the Foyer floor all you need to draw is the roof (insert By Picking Points) so that it matches the walls of your foyer - this will then appear to sit on your exisiting foyer walls. HOWEVER:....
Following Robert's comments about invisible walls and your comment about it "being drafty" I looked a bit closer at the foyer and it has many issues. I would agree with Robert's suggestion that you delete ALL the invisible walls (room dividers). They do not appear to be doing anything but they are probably causing many problems with the existing walls, as some are in the same place as a regular wall (so inside it) and this may be even making some of the walls vanish. You will have to zoom in close on plan view and search around as there are many long and small pieces all over the place. They show up as black broken lines on the plan, (zoom in close). Turn off the floor and Terrain and you will see them easier.
The foyer walls also have many bad joins and broken segments, so I would suggest that they be all redone after deleting the invisible walls and use two locations to get the height (96" for the existing Ground Floor and 48" for the Foyer location as Robert suggested). If the walls is supposed to be 12' inside, then the Foyer location's "floor height" should be 108" (96" + 12" for the ground floor thickness).
To redo the foyer, delete all the existing walls that make it up, then extend the existing walls from the four corners to make the new foyer walls.
Once you have them connected on each corner, shift-select each of the walls that make up the 4 sides of the foyer only, right click and select 'Duplicate to Locations' and select the new Foyer location. That will copy the four walls to the Foyer location and they will be exactly aligned and appear as single wall. As Robert said - turn off the Trim on the upper part of the walls (Foyer Location) and turn off the Drop Face too (set it to zero) or it will overlap the lower part of the wall and may cause texture rendering problems. These upper walls (Foyer location) should also be set to Extend to Roof so they will stop at the roof when it is inserted on the Foyer location.
I noticed too that some parts of the building the floors do not meet correctly or overlap (which will give a two texture conflict). Some of these things may not seem relevent, as I seem to recall that you were mainly interested in the garden, but they MAY cause issues for you when you try to render photorealistic images later.
Hope you can follow all of that and that it makes sense. Ask again if anything is not clear.