Under ground/ basement garages

David Morton

Lifetime Premier Supporter
Any body had experience of building a garage in a basement?
This year I rented an apartment in Italy and the garages were located under the block of apartments. Now I'm back for the winter and part of my plans are a house move in mid November, and the next house is located on the side of a hill. My plan is to excavate the area in front of the next house and build a 3 car garage along with another adjoining basement alongside for house utilities, plonk, and chill room. This is a cunning plan so as to extend part of the house above ground level by demolishing the existing garage.
My question is - has anybody done anything similar and if so, any pertinent advice ?
I have no experience with anything like that, but after reading about the damage caused by Hurricane Katrina, I'd be really careful to make sure that any water that gets into your garage has an easy way to get out.

Other than that, it sounds like a nice idea.

I can tell you from personal experience that unless you have a good French drain system in place and a sump pump with 12 volt backup in case of power failure, DON'T do it! My 40 was nearly inundated twice. /ubbthreads/images/graemlins/shocked.gif Only by sheer luck, was I able to avoid disaster. A GT40 with 4" of ground
clearance, sitting in 3" of water, isn't a pretty sight!
Needless to say, I now have the aforementioned precautions in place.


David Morton

Lifetime Premier Supporter
Thanks for the advice. The property is approximately 400 ft above the River Thames and is extremely well drained with a chalk subsoil base so flooding is not a concern though the Utilities side of the house will have a sump/pump system to help gravity anyway. My -40 lives somewhere else where blondes cannot back into it but I hear the warning.
Dave M

Joe T

New Member
My 4 car garage is built so that the roof of the garage is about half a meter above ground level. I then use the pitched roof area above as an office.
The sides and base are reinforced concrete. I did it this way to adhere to the 4m height restriction.
I had to get an Architect to calculate wall thickness and design.
Its also tanked out with a 1m square hole sunk down 2M from the garage floor with 2 pumps one powered by a solar panel-battery-invertor setup.
I was also advised to membrane the floor and tile it.
Mine's built on clay soil.

I agree with most of what has been said. I would add that there are other considerations to a basement garage. Some of them serve double duty in that they have to house more than our hobbies. A lot of them have to hold the maintenance items for the house, home, and yard. If yours is that kind, think of hanging everything and anything that you can. It will give you instant room to work. Nothing like hitting youir shin on the lawn implements as you are moving about under the car. Also utilise all the overhead space you can to store large bulky items like plywood etc. I have a Truss system of joist and you have to be a little crteative in getting pieces up there. For over a year I suspended the rear and front hatches of my car from those joist with a system of 2x4s and ropes. A high ceiling helps.

Also, cover everything. You won't believe how much dirt and dust will accumulate. The air pressure inside an open garage with no true ventilation, is less than the air pressure just outside of it. Leaves falling by an open garage will be sucked into that open door and can be blown further back than you can imagine. On another note, You are lucky if you don't have clay soils in your area. Clay soils when soaked with water can exert forces you won't believe. In the finished area of my basement those same 8' walls, that are a foot thick, on the up side of a down sloping lot with only 4' of dirt, produced this after 14 years. The original crack was sealed with an injection system. The crack extends the full 8'.

My last suggestion or two is,,, first, install an alarm system to alert you of water in the basement. Anything from a symple alarm That lays on the ground and is activated by the water, if you live in the building, to a system that will notify you via phone or computer if water accumulates. The last suggestion is, that if power outages are not a problem, install the sump pump with two pumps. One a little higher than the first at a level that will not trigger when the first does, but low enough that the sump doesnot fill to the top to trigger. That way when the first fails for whatever reason, the second one will continue to do the job. You might install a sytem that will let you know that the second pump has been triggered. A simple one that say, triggers a light to stay on once triggered would work and let you know that the first has failed. It would have saved me a weekends work with the shop vac once.



Active Member
I built one with a 1/2 exposure to below grade considerations. Use a lot of small rebar. I made tubs and columns out of galvanized sheet metal and then fillied them with concrete.
My ceiling is another slab. Good thing to have fire protection if your garage is under your house.
There are enough original parts in my house though that it still qualifys as a house.


Howard Jones

Gold Supporter
Think about fuel vapor. Gas vapor is heavy and will drop to the floor and stay there until it fills the complete volume of the space diplacing all the air. I think you must actively vent the basement. Maybe put a timer on a blower or look into a fuel vapor sensor at a boat store or other failsafe ideas. Or all the ideas you can come up with together.

In any case I am sure you know what the result of your basement full of gas vapor and the electronic starter on the waterheater will be some cold winter night at 3AM.

I'm currently working on a basement, it's very difficult to sum up in a few words, but there is nothing too scientific about the construction. Bear in mind it'll probably consist of lots of steel mesh, re-bar, a few lorry loads of premix & blocks, & some tanking materials.

Getting rid of a few hundred tons of topsoil can be expensive unless you know someone who needs some.

If you have a friendly local structural engineer it may pay you to have an informal chat.

if I can help more please contact me offline!

BTW, no-one has a basement as cool as Batman!


David Morton

Lifetime Premier Supporter
Thank you everybody for some valuable input for this project.
The challenge is to build it with enough strength so that the driveway to the upper area of the house will be strong enough to park cars and reasonably heavy other vechiles
on top once the drive is put back over the underground garage. The underground garage will exit about 25 to 30m down the hill where there is another gated entrance.
The comments about 'French' drains, ventilation and various alarms are especially noted. Thank you everybody.
Dave M

David Morton

Lifetime Premier Supporter
amazing. i wish.......
trying to use a japanese keyboard here without much success.
if yuji comes on the forum telephone numbers changing from tomorrow - tuesday
nborman, thanks - lots of additional ideas there - i would need to work 6 every year instead of 3 to pay for them though.

Charlie Farley

Sponsoring Vendor
Nothing has changed much since the 'DENSE' years of the 1980's.
Looking at that abortion just reinforces that opinion.
On a cost analysis, it would work out as £10,000 per hour used.
And to reinforce my opinion, look at the predominant colours used, Red and Black, same colours as fashionable quilt covers and interior decor colours of the 1980's.
What lack of imagination in it all.
To the 'desigher' sic, what year is it?