creating a suspended floor

[Burnz0]

Hi All. I've got a 1910-1920s terrace with suspended floors throughout except for the kitchen which was a concrete slab. After panicking about potential damp issues I've decided to go down the route of digging out the slab and fitting a new suspended floor - it will make things easier when i come to fit the final floor covering as the rest of the room (after a knock through) is suspended floor.
I've spent today digging the floor out and have gone down 2 courses of bricks and have now reached the soil under the hardcore level. I'd like to know how much further i need to go down as if i was to match the rest of the house i'd be looking at 3ft or so which is a massive amount of soil to get rid of when you've not hired a skip!
looking on the internet tonight ive read that digging down too far could compromise the foundations? - the kitchen floor level is 3ft from the ground level outside so i guess the foundations need to go down to at least this point - can anyone confirm if my thinking is right?

finally, i'm not sure what i should do about ventilation. The rest of the house is ventilated by an airbrick under the living room and one under the dining room at the back of the house. Since the house is on a slope, the front door, which is on the kitchen side of the house, is only a brick or so above the ground level - the right side of the house is about 1 foot above. Therefore, i don't know if it will be possible to put in an airbrick here? if i put one in the back and poke some holes in the internal wall foundations will there be enough air getting through or does it have to be a cross-flow?

 

[noseall]

Get some floor insulation, some 1200 gauge dpm, some all in ballast and cast a concrete floor.

No need to worry about venting or damp.

 

[Burnz0]

is that actually true though? ive read that (potentially) all you'd do is stop the moisture coming up from the floor and instead it would find its way up the walls?
my preference is the suspended floor as it seems like a significantly cheaper option and will also help me in that ill be left with floorboards in both parts of the room making it easier to lay the final floor covering

 

[stuart45]

is that actually true though? ive read that (potentially) all you'd do is stop the moisture coming up from the floor and instead it would find its way up the walls?

That's only a possibility in an old house with no DPC in the walls.
A lot of older houses with timber floors still had a concrete kitchen floor.
However if you still want to put a timber floor in ideally you need to dig down enough to allow a min of 6 inches between the top of the concrete oversite and the bottom of the joists.

 

[Burnz0]

yeah, that's what's mine has... I can't understand why they'd fit a dpc for the rest of the house and then undo all the hard work by building the concrete base up to the same level with no dpc therefore opening up for the potential for moisture to get past the dpc.
I've dug down about 5 bricks worth so far which should hopefully be enough. Upon closer inspection it appears that the sill plate? is completely rotten - in some places it turns to dust if you rub it slightly - which is of some concern.
When you say concrete oversite - what does this mean? an actual concrete base under the void?

 

[stuart45]

When you say concrete oversite - what does this mean? an actual concrete base under the void?

That's right. Houses used to have just earth below the joists, but since the 20's or 30's a concrete oversite has been put in to help stop ponding and weeds growing. The sleepers walls can be built off it if needed.
It's best if the oversite is at least as high as the ground level.