Foundation depth for single story extension

How deep do foundations for a single story extension on the back of a two story terraced house have to go? I've heard from one person that it depends on whether the extension is one or two story and from another that they must start at the same depth as the house that is being extended regardless of how high the extension is..which is correct? any opinions much appreciated....

40 to 50 cm approx. Got mine done in new ext. a few weeks ago. Thats what I recall they were.

40 to 50 cm approx. Got mine done in new ext. a few weeks ago. Thats what I recall they were.

Yea, mine was about the same. If you're building a 2 storey extension then the foundations have to be deeper to support the extra load.

Ruby, I hope your engineer would know this or whoever did the plans for the house.With respect, I definitely would not make a decision based on what other people on a forum have done.

Houses have been built with no foundations in this country and are still standing. I'd say myself 300x600mm would be fine but you can make it a bit wider and throw in a bit of steel for luck!

Thanks everyone. Con, The architect and the builder and an external engineer have all agreed that the foundations should go down to the depth of the original two story house but those are very very deep (about 2.5m). They say that this is because the ground is very soft in the area. Despite the experts I am worried and was still wondering whether a single story should have to go as deep as the original foundations.

if they say so, I'd go with them rather than anonymous posters on the web who don't know the details like the architect AND builder AND engineer.

Remember, the builder might make more money outta it but it means nothing extra to the architect or the engineer so why would they lie ?

Also, going to the same depth as the original will give you the option to put a second floor onto the extension in the future.

the foundations are made by pouring concrete (25 or 30 newton ) into a trench and is usually a foot deep and three feet wide. how deep the trench depends on the ground-there is no point in pouring concrete on soft earth so you dig trench until you hit hard ground,then pour the concrete with rebars buried inside the concrete.you then use blocks to bring it up to where you want your floor level to be-i presume the same level as the house

Refer to Part 5 of Build Regulation 1997 if you can.

If you cannot, strip foundation specs specify by Homebond are
- Min Foundation width = 3 * wall tickness
- Bottom of foundation min 600mm from ground level
- Concrete tickness min 300mm

Ruby -

Are you posting to get reasons to overrule your architect and engineer? Because that's how it looks!

If they say the ground is soft, and they *all* agree the foundations need to be deep, then I strongly recommend taking their advice. The building regs prescribe minimum requirements for normal conditions, but there are all sorts of ground conditions that'll require more... if you overrule them, you really don't want to find yourself realising, in five years' time, that the architect was right after all.

As the proud possessor of one "lean-away" extension (pending demolition, and pre-dated my purchase of the house) I can tell you that if you don't get the proper ground works done there's almost nothing you can do
short of demolishing and re-building.

my experience when we built our extension (which is also on semi-soft ground) was to dig down to level of foundations of house (approx 9 feet in our case) , backfill the trench with approx 300 mm leanmix (approx 15 newton dry-fill, compacted with whacker) and then pour proper foundations on top with mesh re-bar - this on the advise of our engineer - said the last thing you want is uneven settlement and for the extension to peel away from the main house !