Digging the hole

[enahs]

I would like to have a 3mx3m side extension built that is strong enough to withstand an additional room, eg a bathroom some years down the line. I would like to know how deep the footings should be and how to tell if it is safe to dig out the earth without upsetting the existing foundations of my house. Thank you.

[mjpliv]

Footing depth depends on a few factors. One of these is frost. We would need to know where you are building the addition. You mentioned the 3M by 3M dimension so I am guessing Canada.

[Rich]

I'm going to guess the UK :)
Looked at email

[mjpliv]

:P

[enahs]

Hi guys, I live in Portsmouth UK in a semi-detached house and want to build up to the central point/joining wall of both houses in the terrace.

[mjpliv]

If you are building in the UK I would doubt that there are any real frost issues to deal with.

If this is an older home I would be half tempted to explore the possibility of using a structural slab poured on free draining material with steel ties epoxied into the top of the existing foundation.

Alternately, the traditional footing and foundation wall would be fine. You should not have any concerns about excavating down as far as you need to. In fact, I would suggest that the footings adjacent to the house be at the same level, and be dowelled to, the existing footings. The reason for this is evident when you look at the possibility of any settlement of the new structure. If the structure drops on the edge farthest from the existing home one of two things will happen - either the upper reaches will pull away from the existing home or the footings will push against the existing foundation. If the footings are placed against a foundation wall it can cause them to deform inward. If the new footings are against the original footings then there is little chance of them moving. Once you move away from the original wall you can step up the footings if you do not require the depth.

[mjpliv]

Another note - Fire Separation!

If you are building a structure that extends along your property line at the juncture of the two homes you should check with whatever building authority applies with regard to what fire separations are required. I would assume that there will be requirements for non-combustable materials as well as a requirement to have no unprotected openings (ie - soffits, windows, vents, etc) along that wall face. This protects both you and your neighbor.

I would actually be interested in finding out if this rule does apply in other countries.