Floor level, new footings

[Hope_floats]

Hi, I'm just starting digging the footings for my 2 story extension and wanted advise on matching old house floor to new.
Current regs require the new floor to be total thickness from the footings of 420mm, sub base, insulation etc. The old house floor is only 220mm from footings to finished floor level.

My question, do I need to have my footings (1mtr deep) to be 200mm below the existing footings?

Does that make sense?

 

[^woody^]

Does that make sense?

No.

The floor and footings are not related.

Footings (ie foundations) are the bits of wall normally on concrete below the ground and have nothing to do with the floor.

 

[Hope_floats]

Sorry, as you can tell, I'm new to the terminology
I have drawings that show I must make the extension internal floor up to finished level using compacted sub base, sand, insulation and a final 75mm screed.

I must trench fill concrete foundations to a minimum depth of 1mtr, and goes on to show the flooring requirements (inside of the brickwork) that show the ground above the concrete to the finished floor level is a minimum of 420mm.

The old house I'm joining to, from the concrete to finished floor level is only 220mm. If I concrete to the same level as those concrete foundations, I'll have a new floor 200mm higher than the existing floor.

Should I then dig out another 200mm of the internal area to give the required depth of the new base of 420mm? Or should I start the 1mtr concrete foundation level, 420mm lower than the existing finished floor height?

 

[Notch7]

I think you are getting confused.
It sounds like you have building regs drawings with cut n paste foundation detail.
Ignore the dimensions on those.

Just go by this:
Foundation
Typically bottom of foundation trench is around 1 metre below ground level.
But the actual depth on your project needs to be when stable, load bearing ground is reached. Depth is also influenced by nearby trees or drain runs.

When the foundation concrete is poured the top of the top of the concrete should line up with one of the brickwork courses on the house. Ideally you will want to be to have a number of brick or block courses from top of foundation to dpc level.

Typically from dpc, you will gave 3 courses of matching face brick. That allows the 2 courses of face brick above ground, then 1 extra to ensure you only see face brick above ground.

Anything not visible can be concrete block or suitable brick (engineering are good).

Floor make up
Ignore foundations.
The only thing you need to know is do you reduce dig so the distance from existing finished floor level down to ground is the same as your total floor make up.

Ie hardcore + sand + dpm + concrete slab + insulation + screed

Nb: that assumes ground is stable at that depth.

 

[^woody^]

Or just ....

Scrape the top 250mm or so (all the soft top soil) off the whole extension footprint.

Dig the trench 1m deep.

Fill with concrete and build wall up to DPC level

Measure down from the top of the wall the depth of your floor, insulation and hardcore.

 

[Hope_floats]

Gents

Thank you very much for the help, the top ground is poor (lots of old bricks and clay) so I'll take the whole footprint down and do it that way.

Thanks again, I'm sure I'll no doubt have more questions as this project develops.

 

[Leofric]

The foundations need to be approved by Building Control anyway .