converting crawlspace to basement questions (floor, how much, foundation, room)

[Dea13]

Hello,

our house was built in 1959 and has a full basement under it. An addition was put on in the '70s and has only a crawlspace (about 2 ft high). Total sq ft of house is 1483 and the crawlspace is roughly 600 sq ft of that. I'd love to have the crawlspace made into basement.

I'd appreciate info on the following (yes, I'm as clueless on where to start as the questions sound):

1. What would I look under in the Yellow Pages to find the type of contractor to do this?

2. What types of questions should I ask?

3. Anyone have a guess on what this would cost? Just looking for a ballpark range.

4. Would this negatively impact how waterproof the existing basement is where the walls would connect?

Thanks for any info.

Dea

[Cosmic]

Yes, you can back fit a basement under an addition. Not be best way to get a basement. Probably might not get it all from one guy.

First you have to give the new addition some total support. Something along the line of some heavy wood or steel beams running across underneath in parallel with the main house. They have to stick out a long ways pass the addition, like 8 - 10 feet on both sides to give working room. Somebody like a house moving company is well suited to do this type support.

Then you dig out underneath a big hole like most foundations, only it will require smaller equipment that can maneuver in the area. Once you got the hole, probably want to pour the basement floor first.

Probably want to go cement block for the walls. Going to be less steps and be cheaper. You can just lay the block up and leave holes for the support beams, transfer the load to the block walls in places, remove the support beams. Finish up the wall lower the addition completely in place.

Always difficult to guess what a job like that might take money wise, varies so much by location. Maybe $20K range. Somehow you also have to cut out an opening in the existing wall for a new door into the new basement section.

The seal between the existing walls and new should not be an issue, there are ways to doing it pretty leak proof.

[Barking Spider]

There was an article in Fine Homebuilding last year about someone that did that. It was a costly project for them, but they felt the investment was worth it.

Costs obviously are determined by what the prevailing labor rate is in your area, and how hard the job is. Obviously you would want a foundation contractor, or a General that had a good grasp on this kind of work.

[harry chickpea]

Ohferpetesake. Cosmic, you've read how I worked on something like this in my parents house.

Our basement happened to be a basement/garage. We knocked a hole through the wall of the basement on the side of the crawl space and immediately framed that in to prevent shifting.

The idea was to dig as much dirt out as possible, while still leaving about 2' of dirt near the footings of the crawlspace foundation UNDISTURBED. We used a mattock/pick and a tiny riding lawnmower and cart to remove the dirt. Once all the required dirt was removed and we had dug down for a sub-basement drainage system, the contractor came in. He put in crushed gravel and a sump pit, set the floor, put in an amazing amount of rebar, set forms, and made concrete walls that went just above the existing footers of the crawl space and were tied in to them. The issue is that there was about a 3' shelf halfway up the wall, formed by the lower wall and concrete surface that merged with the upper wall. The project was dirt cheap because of our cheap labor.

A addition of the basement like this is not rocket science. If you are willing to put up with slightly less area it is inexpensive.

If someone wants, after the fact to rethink an addition and put a full absement underneath, then yes, some extreme measures need to be taken. To gain some extra space, that isn't needed.

[Triadhandyman]

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Originally Posted by Dea13

Hello,

our house was built in 1959 and has a full basement under it. An addition was put on in the '70s and has only a crawlspace (about 2 ft high). Total sq ft of house is 1483 and the crawlspace is roughly 600 sq ft of that. I'd love to have the crawlspace made into basement.

I'd appreciate info on the following (yes, I'm as clueless on where to start as the questions sound):

1. What would I look under in the Yellow Pages to find the type of contractor to do this?

2. What types of questions should I ask?

3. Anyone have a guess on what this would cost? Just looking for a ballpark range.

4. Would this negatively impact how waterproof the existing basement is where the walls would connect?

Thanks for any info.

Dea

[Dea13]

Thanks everyone. I suppose questions I should ask are if the company is licensed and insured, if they have some kind of price guarantee to avoid low bids with huge overruns, what services/materials are included in the bid. Guess I'll start with looking in the phonebook under general contractors and see where that gets me.

We don't plan on doing any of the work ourselves. My husband is recovering from a boating mishap and is looking at having surgery on his elbow so he's out of commission.

Dea

[M3 Mitch]

One big question will be how to get the dirt you remove out, and how much of a mess the digging will make in the existing basement. If you have to haul the dirt out by hand this will be a labor intensive project.

[Cosmic]

Originally Posted by M3 Mitch

One big question will be how to get the dirt you remove out, and how much of a mess the digging will make in the existing basement. If you have to haul the dirt out by hand this will be a labor intensive project.

The type of dirt is also going to determine a lot of it. Is it easy digging or tough olde clay. That same thing that came up in some other threads. The Angle of Repose or the natural angle that the soil will support itself. In some soils you may have to do a lot of bracing to prevent the sides of the hole collapsing as it is dug or dig way back at shallow angles into the hole. Maybe even a type of pilings / lagging like used in industrial applications to dig big deep holes in urban environments.

Not really the type job I would like as a contractor. Far to many risky variables that can not be fully predicted in advance. Lots of chances for Murphy's Laws to come into effect.

So, so much easier if you dig the hole first, build the basement and then put the new addition on top of it like normal.

[Dea13]

M3 Mitch & Cosmic - Thanks for the replies. I didn't think about how the type of dirt could impact. I can see now how that could cause problems. We have very hard clay. We actually brought in soil when we made our garden. When our soil is dry it's like cement! Not sure if that's good or bad.

In my head I was imaging a bobcat (my daughter always called them jobcats) from the back or side of the house digging a trench and big hole going under that part of the house and putting supports in as it dug. Then backfilling the big trench in the yard that created. I realize I'm probably way oversimplifying. Also would have expenses for cement, walls/supports and wiring. Also, we have pipes/ducts running through the crawlspace so plumbing and possibly some ductwork would need to be done. Would that be all the pieces or am I missing something?

If I had lived there when the addition was put on I definitely would have had the full basement done but the addition was done over 20 yrs before we bought the house. My high end for deciding to do it will probably be around $35k. I'm hoping it will help with some allergy problems. Also, with the extra space I can stop renting a storage unit and finally be able to park in our garage!

We make a habit of going through our basement/garage every year and if something hasn't been used in 5 yrs we get rid of it. It's amazing how much room is consumed by our dogs, hunting/fishing/camping stuff and just house items like lawnmower, snowblower, rakes, gardening items, etc.

Dea

[Cosmic]

Originally Posted by Dea13

M3 Mitch & Cosmic - Thanks for the replies. I didn't think about how the type of dirt could impact. I can see now how that could cause problems. We have very hard clay. We actually brought in soil when we made our garden. When our soil is dry it's like cement! Not sure if that's good or bad.

In my head I was imaging a bobcat (my daughter always called them jobcats) from the back or side of the house digging a trench and big hole going under that part of the house and putting supports in as it dug. Then backfilling the big trench in the yard that created. I realize I'm probably way oversimplifying. Also would have expenses for cement, walls/supports and wiring. Also, we have pipes/ducts running through the crawlspace so plumbing and possibly some ductwork would need to be done. Would that be all the pieces or am I missing something?

If I had lived there when the addition was put on I definitely would have had the full basement done but the addition was done over 20 yrs before we bought the house. My high end for deciding to do it will probably be around $35k. I'm hoping it will help with some allergy problems. Also, with the extra space I can stop renting a storage unit and finally be able to park in our garage!

We make a habit of going through our basement/garage every year and if something hasn't been used in 5 yrs we get rid of it. It's amazing how much room is consumed by our dogs, hunting/fishing/camping stuff and just house items like lawnmower, snowblower, rakes, gardening items, etc.

Dea

I don't think you can get away with that type of support system. Even if you use a bobcat, it will have to maneuver without any possible chance of hitting supports.

Again your best option is going to be putting in very long steel beams to pick up and carry the load completely giving as free an access space around the site. You want the steel beams to stick out a lot of both sides and be on firm bearing surface on the sides of the hole, probably lagged in some fashion to prevent collapse as it is dug.

I doubt poured concrete walls are an option, not enough vertical clearance over the tops of forms, maybe a type of precast panels, but cement block would probably work best. Allows you to build up and around the support beams, leaving clearances around them, transfer the load to the new walls in sections while extracting the support beam, finishing off the walls as you go.

You can't get into situations where the work space is too cluttered with supports and you can't mess around trying to shift load during the job. Would take a big amount of cribbing, bracing and supports if you try to do from underneath as a post type system. Safety becomes a major aspect of the entire process.