Basement under screened in porch (foundation, drains, installed, room)

[618719]

My house is on a slab and yard on a slope to the back.

I'm thinking about having a screened porch put in off the back of the house. Roughly 20 feet away from the back of the house, ground level is about 8 feet below the slab.

That combined with the lack of room in the house and garage for storage and a workshop gave me the idea of having a basement put under the porch for this sort of use. Not connected to the rest of the house interior, but with a walk out door to the yard on the back side.

Never really seen anything like this though, so thought I'd see if any of you had immediate thoughts or comments. Great idea? Horrible idea? Can you dig out 8 feet of dirt adjacent to the edge of the house without extremely expensive structural work? If not, how close could I get? Say the porch sticks out 16 feet and I make the basement area only the outer 10 feet.

Would something like this add much value to a house or am I going to lose a lot of the money I put into it? If so... maybe just renting a storage unit would be better.

[chet everett]

While it is generally an excellent idea to to have a solid foundation even for things like a porch the issue that you run into when trying to create a "cellar" that is not connected to the existing foundation is risk vs reward.

It would be one thing if you said you wanted to expand the existing house and tie the new foundation into the existing basement I would say that is a solid idea -- it allows for the easiest connections for utilities and such. The waterproofing would of course have to be done to the same standards as the existing home regardless of whether you tied it into the existing foundation as would not want water under new living space. What you are proposing is different -- instead of just pouring footings and then building a simple block foundation to the support the porch (which is the accepted construction method) you want a full height basement that would be essentially a "cold cellar". The side entry door, or even a bulkhead style cellar entry is not too big a deal BUT to keep the space water tight the whole foundation extension should have perimeter drains installed. If you have existing sump pit and pump that should be tied into the new foundation protection. If there is no sump pit you'd need to plan for that in the new space, which would mean additional connections for utilities and discharge.

A properly constructed home should NOT be adversely impacted by having the space adjacent to the foundation excavated but the depending on the conditions of the home / changes from when it was initially built there is always the POTENTIAL for improper construction techniques to be exposed by such digging -- if you want more info you will need to talk to qualified local professionals that can determine the feasibility of leaving things as they are or making necessary repairs.

The costs for not just rebuilding the porch but also fixing any issues that would be revealed by excavation and then digging out a full cellar could easily make the porch prohibitively expensive -- depending on local charges perhaps more than double. If you do not care about the cost and are doing this for your own ongoing use then go right ahead, but if you hope to recover your expenses when you sell this is probably not the best use of your money -- if it is not done properly you will end up with some freaky "bunker". A few weeks back another poster on the forum ran into a situation where that sort of unusual underground structure created more headaches!