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Sanity Check - Digging out under a single car garage

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  • Aug 27th, 2021 12:02 pm
[OP]
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Dec 8, 2008
325 posts
69 upvotes

Sanity Check - Digging out under a single car garage

Looking for some opinions if I’m out to lunch on a project being planned or if it makes (some) sense.

My partner and I bought an 1960s SFH in 2018 and have been slowly updating it. 4 bed 2 bath, roughly 1600 Sq ft. It wasn't necessarily going to be our forever home but we've settled in and don't know where else we would go given the way housing prices have gone.

We're currently in the planning stages to finish the basement. The previous owner had a woodshop in the basement, and made the unique architectural decision to dig out a staircase in the garage floor that runs down into the basement. It is an attached one car garage, and as a result it is too narrow to park a car in there.

I'm considering three options and would appreciate some feedback:

1. Fill the staircase, wall in the door to the basement
  • This would give us a functional one car garage. I have gotten a very rough quote from a friend in the foundation business, and he said to do this turnkey would be slightly over $10K (including removal, finishing, etc.).

2. Dig the garage floor out, block out the new walls underneath and add a wood flooring system above
  • This would give us a non-functional garage, essentially making it a large shed as the floor wouldn’t be able to support a car. But we would add a couple hundred square feet of living space to the basement. Foundation friend proposed two options: 1) leave the existing staircase and build around it, or 2) dig out the staircase as well ($$$) to make the new basement room slightly larger. Leaving the existing staircase and just digging out the floor he estimated to be around $10K as well, on top of the cost to finish it with a ceiling/floor.

3. Dig the garage floor out, block out the new walls, and add reinforcements to pour a new concrete slab.
  • Best of both worlds, but likely cost-prohibitive. Would need a structural engineer to come in to draw up plans, involves a ton of steel and estimate was in the tens of thousands of $$ range.

I'm leaning to moving forward with option 2, but wanted to see if this board has some sober second thoughts. Option 1 is the most practical, but would only allow me to park a single car in the garage. We are generally short on storage writ large, and would likely end up using the garage as storage anyway. Most of the homes in my neighborhood are SFHs with one car garages, and most people park in their driveway.

I am somewhat concerned with resale, but also adding living space is quite nice for a growing family. Happy to add pictures or more info if constructive. Have I lost the plot – how crazy is it to renovate a non-functional garage into still a non-functional garage?
3 replies
Deal Addict
Jun 26, 2019
1993 posts
1723 upvotes
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alfietopshelfie wrote: I'm considering three options and would appreciate some feedback:

1. Fill the staircase, wall in the door to the basement
  • This would give us a functional one car garage. I have gotten a very rough quote from a friend in the foundation business, and he said to do this turnkey would be slightly over $10K (including removal, finishing, etc.).

2. Dig the garage floor out, block out the new walls underneath and add a wood flooring system above
  • This would give us a non-functional garage, essentially making it a large shed as the floor wouldn’t be able to support a car. But we would add a couple hundred square feet of living space to the basement. Foundation friend proposed two options: 1) leave the existing staircase and build around it, or 2) dig out the staircase as well ($$$) to make the new basement room slightly larger. Leaving the existing staircase and just digging out the floor he estimated to be around $10K as well, on top of the cost to finish it with a ceiling/floor.

3. Dig the garage floor out, block out the new walls, and add reinforcements to pour a new concrete slab.
  • Best of both worlds, but likely cost-prohibitive. Would need a structural engineer to come in to draw up plans, involves a ton of steel and estimate was in the tens of thousands of $$ range.
1. Yes, you pay the least amount of money, and you get back a functional garage, not much to comment on here.

In regards to number 2, I think your friend is greatly underestimating the cost to do this. Did they have an itemized cost breakdown? The footings of your garage, or whatever is holding it up will be completely inadequate and 99.9% likely not go deep enough for a basement. You would have to underpin the whole thing to extend the foundation, and alter the existing footings/foundation as required. I feel as though this work along would probably come close to costing $10k. In fact by the time you get design, permits, etc its a large chunk of that already.

For 3, as it is a single car garage this is less expensive compared to a double car garage. For a number projects I've seen prefab slabs been used and air sealed to span the distant, others have just constructed the slab in place. Again, for this option, you are going to have all the costs of option 2, with added costs of building a beefy slab, foundations/footings might need to be larger as well.

Again, just based on my experience, but I would for sure check your friends math/items on item 2. I think by the time you pay for permits, design, and foundation work, you're over $10k before you even touch anything else. If its just an attached garage with nothing on top, depending on siding and other stuff, it may be cheaper to just knock the whole thing down and start fresh as opposed to underpinning and what not.
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Dec 13, 2003
480 posts
47 upvotes
For number 2 you would also need to include the cost of closing off the garage door and replacing it with a single man door then finishing the exterior of the house. You want to eliminate the possibility of someone driving in if it is not able to support that.
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Apr 26, 2003
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Agree with SubjectivelyObjective that option #2 would be much more expensive. You'd have to underpin the entire garage which would be $$$.

Think about what you actually want/need? Storage or garage space? If you don't plan on parking in the garage, turn it back into a garage for parking/storage. The other options are significant and will cost $$$ which also in the future will yield questions like you have now as to why you did something to the garage and turned it into something that isn't usable for parking.

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