Extension footings depth

[an203888]

Hi all.

I am shorty going to be hiring a digger and digging the footings for my 40m2 rear single story extension. The houses on our close ar all on a slight slope, so on one side of the house it would appear that the top of my house footings are nearly level with the neighbours driveway (maybe 300mm lower) but are 860mm down from my ground level. The footings are about 300mm deep and appear to be built on sand.

What depth would you dig to prior to the building inspector coming out? Please see attached picture to give a better idea based on a test hole I have dug. I have a video in my media but not sure how to add it to this post.

IMG_20191208_093739

• an203888
• 8 Dec 2019

 

[nortonj]

Im sure on specs for my extension is was 300mm depth by 600mm wide.
Building just dug diwn till he found some firm ground, then mass filled them so they ended deeper than the 300mm

 

[an203888]

It's more the slope and BC I'm worried about..

 

[nortonj]

Could you not step your footings?

 

[Notch7]

It's more the slope and BC I'm worried about..

Footings can be stepped.

 

[johnny2007]

From experience I woild ask building control what they want.
For example in brentford (west London) they insist on 1m footing despite finding hoggings at 400mm.
Many builders go down 1m, get the inspection done, then refill the hole with hoggings, but by then they're not compacted anymore.
A bit of a mess.
So, ask BC.

 

[an203888]

From experience I woild ask building control what they want.
For example in brentford (west London) they insist on 1m footing despite finding hoggings at 400mm.
Many builders go down 1m, get the inspection done, then refill the hole with hoggings, but by then they're not compacted anymore.
A bit of a mess.
So, ask BC.

Why on earth would you refill the hole!!
It's not so much the depth from where I'm digging but more the fact that it's kind of on a large slope. Just trying to guage what I should do before I get them to look but yes I am best just to ask them. Thanks

 

[^woody^]

Many builders go down 1m, get the inspection done, then refill the hole with hoggings, but by then they're not compacted anymore.

Wtf. Do they really? You must know some right questionable ̶b̶u̶i̶l̶d̶e̶r̶s̶ cowboys.

Most builders/DIYers/normal people I know dig down to whatever the plans say.

 

[^woody^]

my 40m2 rear single story extension

You've got no plans then?

1m minimum from the lowest ground surface level

 

[an203888]

You've got no plans then?

1m minimum from the lowest ground surface level

I have plans... They say this...

Screenshot_20191208_185202_com.google.android.apps.docs~2

• an203888
• 8 Dec 2019

Also my local building control guide says this -
"Foundations should generally be :-
• A minimum 750mm deep or 1000mm if
shrinkable clay type soil.
• 600mm wide.
Please note : precautions should be made for
nearby trees, drains or poor ground conditions
may also be required, depending upon location."

It's not really about how deep under normal conditions, its how deep when building on stepped ground when the existing floorings are as deep as explained above. I expected them to go down a little further but they don't and just wondered if anyone else had some experience of this.

 

[Leofric]

If you don't know what you are doing I wouldn't dig any more. If your designer can't give any more specific information on the foundations to suit your particular situation ask Building Control to come to site and discuss it with them.

 

[^woody^]

I can't understand how you can have plans approved when they don't even specify a foundation depth.

Or if the designer presumably visited and saw stepped ground.

Stepped ground our not, foundation depth is always taken from the lowest ground level, and foundation bottoms are never stepped, unless they are specialist designed.

 

[paulrockliffe]

I have approved plans that say the same, dig until BC are happy. How can you specify a depth before you've dug, short of getting a rig in to take core samples?

 

[Notch7]

I have plans... They say this..

yes its cut and paste building regs drawings spec

-the really key point is 'All foundations to be to LA approval'.

1 metre depth is considered typical but it entirely depends on load bearing capacity of the soil and any influences like trees.

Sloping sites as mentioned can be dealt with by steps -but these must adhere to rules.

If you are sufficiently confident to use a digger on a sloping site, I wouldve thought you might know how to set out for the foundation -If you arent I wouldnt go too far, if you take too much out you cant put it back!

 

[Leofric]

If you have dug a trial hole to expose your foundations and your neighbours, discuss it with building control on site on how to proceed to suit your particular site conditions. There are rules for stepped foundations under bldg. regs as others say.