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mattdaniels

Original Poster:

6,305 posts

168 months

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Sunday 21st November 2010 quote quote all
Looking to create some more space and one possibility is a larger bedroom with en-suite. This would mean extending over the top of the part-integral garage. I am assuming this would mean having to strengthen the outside garage wall (as it's single skinned) and having deeper footings? How deep are they likely to be and deep do they need to be?

Sam_68

9,939 posts

131 months

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Sunday 21st November 2010 quote quote all
Short answer: as deep as the Building Control Inspector tells you to. smile

Minimum depth is 450mm from finished ground level to underside of foundation (to ensure adequate frost protection), but there can be other influences (as examples, shrinkable clay, disturbed ground or influence of tree roots) and, indeed, some situations where simple concrete strip footings are just not adequate and you need to look at more complex solutions like piles or rafts.

Dig your footings to the appropriate minimum depth, or until you hit what appears to be indisturbed subsoil, whichever is the deeper, then get the BCO out to have a look: the good ones know the ground conditions for their patch like the back of their hand and will tell you at a glance whether you're ok.

DrDeAtH

1,551 posts

118 months

[news] 
Sunday 21st November 2010 quote quote all
Best off getting a structural engineer in ... but you will probably be looking at 2 metres minimum depending on the soil structure.

Sam_68

9,939 posts

131 months

[news] 
Sunday 21st November 2010 quote quote all
DrDeAtH said:
...you will probably be looking at 2 metres minimum depending on the soil structure.
Sorry, but this is plain incorrect - 2 metres would be a very deep strip footing!

And you don't need to engage a Structural Engineer in the first instance - speak to you local Building Control department.

ETA... but the bad news is that it will almost certainly be more economical to simply demolish your existing garage walls and rebuild from scratch, if they are single skin... the foundations won't be wide enough and it's easier to build a new cavity wall to the full height rather than bugger about trying to tie a new skin to the existing wall (which I assume will be stiffened by thicker piers).

...and with respect to foundations, obviously in the majority of instances, the depth and type of your existing foundations will tell you how deep you need to place the new ones.

Edited by Sam_68 on Sunday 21st November 17:12

otherman

1,168 posts

51 months

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Sunday 21st November 2010 quote quote all
DrDeAtH said:
Best off getting a structural engineer in ... but you will probably be looking at 2 metres minimum depending on the soil structure.
I'm a structural engineer and this is madness. Do not attempt to dig 6 foot deep trenches in your garden / near your house. Just in case you take this seriously for some reason.
As the poster above says, you'll need to be around 18 inches (aka 450) as long as you've hit good formation.
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m4ckg

553 posts

77 months

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Sunday 21st November 2010 quote quote all
Sam_68 said:
DrDeAtH said:
...you will probably be looking at 2 metres minimum depending on the soil structure.
Sorry, but this is plain incorrect - 2 metres would be a very deep strip footing!

And you don't need to engage a Structural Engineer in the first instance - speak to you local Building Control department.
+1 I've looked into a similar job for a friend and was advised by building control that the footings will be to the depth of the existing house

mattdaniels

Original Poster:

6,305 posts

168 months

[news] 
Sunday 21st November 2010 quote quote all
Thanks everyone.

Sam_68 said:
it will almost certainly be more economical to simply demolish your existing garage walls and rebuild from scratch
I think you've actually answered the question I wanted answering but didn't actually ask rofl

Edited by mattdaniels on Sunday 21st November 17:24

GuinnessMK

1,244 posts

108 months

[news] 
Sunday 21st November 2010 quote quote all
m4ckg said:
Sam_68 said:
DrDeAtH said:
...you will probably be looking at 2 metres minimum depending on the soil structure.
Sorry, but this is plain incorrect - 2 metres would be a very deep strip footing!

And you don't need to engage a Structural Engineer in the first instance - speak to you local Building Control department.
+1 I've looked into a similar job for a friend and was advised by building control that the footings will be to the depth of the existing house
There is no hard and fast rule. At all.

The depth of the foundations will be dependant on numerous factors, including the ground conditions, neighbouring structures, the type of foundations proposed, the type of extension being put up etc etc.

As said, contact building control in the first instance. They may have an idea, but until you do some investigation of your own site, you won't know. I've seen foundation depths vary as a result of changing ground conditions across a single site, let alone along an entire street.

Our existing house is a Victorian Terrace, sat on about half of nothing as a found, no mass poured concrete, nothing substantial at all really. We put a brick skinned timber framed single storey extension to house a downstairs toilet and utility room out the back. Building control officer wanted 1m deep foundations (until he was "persuaded" otherwise wink ).

Edited by GuinnessMK on Sunday 21st November 17:33

Sam_68

9,939 posts

131 months

[news] 
Sunday 21st November 2010 quote quote all
GuinnessMK said:
There is no hard and fast rule. ...I've seen foundation depths vary as a result of changing ground conditions across a single site, let alone along an entire street.

...Our existing house is a Victorian Terrace, sat on about 12" of brickwork as a found, no mass poured concrete, nothing substantial at all really.
All true, and, to be fair, I've come across sites where we've demolished houses that have been happily sitting on conventional strip footings since the 1930's with no ill effects, but where we've had to design rafts for the houses we've replaced them with, due to intervening changes in ground conditions, but no point in getting too paranoid!

As an example of how ground conditions can change across one site - and of how familiar a good BCO is with his patch - I remember years ago when I was working in a junior position in a Local Authority Building Control department, we got an application in for an extension. 'I want to go have a look at that', says my boss, 'there's a fault in the underlying rock strata runs through that area and I reckon it could be bang under where they want to put that extension'. And he was right... literally to the metre, on an area where the housing had been up for maybe 60 years.

mattdaniels

Original Poster:

6,305 posts

168 months

[news] 
Sunday 21st November 2010 quote quote all
Thanks again for the information chaps.

Have never had an extension built before, so whilst we have an idea of what we want, we need to sit down with someone who knows what's allowed, or better still can take requirements and make suggestions based on compromising what we'd like with what we're allowed to do.

Is that what an architect does or do they just draw the plans based on telling them what you want?

Can anyone recommend someone in the Northampton area we can talk to? We're trying to do the legwork now with a view to building in the spring.

Piglet

6,061 posts

141 months

[news] 
Sunday 21st November 2010 quote quote all
We've just (well nearly!) finished a two storey extension, self built with some professional input for blockwork and some of the more tricky sections.

Be slightly aware that if you've only got a garage width, by the time you've got blockwork with a cavity you won't have a huge amount of room space. We've created a self contained annexe with not much more width than a garage width (it was our parking space) but about three times as long. It's not huge, but its good enough to serve our purpose.

Spudler

3,048 posts

82 months

[news] 
Sunday 21st November 2010 quote quote all
mattdaniels said:
How deep are they likely to be and deep do they need to be?
In my neck of the woods it will never be less than a meter deep, not a hope in hell would i get away with a min of 450mm.
Maybe in your area it will be different...wouldn't bank on it tho.

Slagathore

3,970 posts

78 months

[news] 
Sunday 21st November 2010 quote quote all
Spudler said:
mattdaniels said:
How deep are they likely to be and deep do they need to be?
In my neck of the woods it will never be less than a meter deep, not a hope in hell would i get away with a min of 450mm.
Maybe in your area it will be different...wouldn't bank on it tho.
Same round here.

Never seen less than a metre on a strip foundation. Would always go down to a metre deep as standard, then building inspector would come and have a look and say if it was OK or needed to go deeper.

Done slightly less before for a pad foundation that was taking a steel column.


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