Footing course width? for 276mm cavity wall + plinth

Hi,

Still working on the design for my extension. The footings have been poured (450mm wide). Now I'm looking at laying the first course of concrete blocks. I have the original house build drawings which state a 276mm cavity wall with 25mm wall insulation. As the first few courses of the walls are stepped out by 50mm, the total wall width at the first few courses is 325mm. I have 100mm aggregate blocks for the first couple of foundation courses, but as I need a width of at least 325mm, how should I construct these first courses. I plan on having a block and beam floor restings on the inside course.

I could just lay the aggregate block courses with a 125mm cavity and infill this, or even lay 3 blocks side by side. Or maybe I should have bought the wider foundation blocks?

Any help appreciated, thanks.

600mm wide concrete footing will be required.

450mm wide footings are for 9" solid walls.

thanks but 450 is fine for a single story building in firm clay....

As you already have the blocks I would build the first few up in 13 inch solid. Rather than lay 3 side by side, lay 2 flat and 1 along side them, and then the next course lay the 2 flat ones over the other side and the other one along side so that it bonds in better.
When you are closer to DPC either use 6 inch blocks or cut the bricks to form the base for the plinth, set out the cavity width and the internal wall.

thanks, but I've already layed the inside course of blocks. So was thinking about just laying the 2 outside courses face down now and tieing into inside course with wall ties, leaving a 75mm cavity? to match the house.