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Tearing down existing home and building on the same foundation

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Posted by: from Pickering
2/15/2020 at 12:45:40 AM

Hello,

I currently live in a home built in the 1980s. The ceiling heights of the home are 7.5ft basement, 8 foot main and 8 ft on the second story.

We have considered doing a tear down of our current home but like the shape and size of it. We want to build a new custom home with 9 ft. basement, 10 ft. main and 10 ft second floor ceiling heights (and potentially a great room with vaulted ceilings).

Is it reasonable to use the existing foundation and re-build on top of it? I assume we would have to add to the height of the current foundation. Would this be easy to do or even possible? Or is it better to just start from scratch and re-do the foundation as well.

Thank you!

REPLIES (15)
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Date/Time2/15/2020 at 3:08:28 PM

Is hard to say without a proper structural test.

Structural engineer needed to determine if the soil under the foundation and the foundation itself is solid enough for the extra structure weight which the new structure will add.

So it is not just a matter of asking if it would be OK to add few more feet of structure.. If anything give way after your house is build, it could cause small to major issue to your new home..

So sometime is easier to just knock it down, reassert the soil and rebuilt to spec of the engineer.

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Date/Time2/15/2020 at 3:09:26 PM

Hello John,

Is it possible to send me some pictures from basement and from exterior of your house then I can give you an idea. if you have a survey please send it to me as well.

Thank you.

Pi-Pa Design and Construction

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Date/Time2/15/2020 at 3:11:09 PM

Hello,

Yes, this is possible.

I am currently building a home that we have done exactly what you describe below. There is some additional engineering work and some finesse involved in the demolition, but the costs for a new foundation exceed this.

Is your basement finished? If it is, it will be difficult to maintain it and you will need to be prepared to redo it as well.

If you need any more info, feel free to reach out.

Justin 647-884-3550

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Date/Time2/15/2020 at 3:26:40 PM

OK,

first of all, the height of the foundation has little relationship to the height of the basement. Engineered floor trusses can be used to make the height of the basement to the level you want, assuming the foundation is sound.

The increase in weight for the addition 2 feet at first and second story is negligible, and has only nominal effect on loading the foundation.

As for the comments about soil below the foundation and footings, well, not sure that guy builds houses, maybe garden sheds ;)

The snow load on your existing roof alone calculates out to about 20X the additional construction material load increase for those additional 2 feet per level.

This is slide rule stuff, and contractors should know better. Since you need engineering done anyway, the trusses, loading, etc., will all be done by your architect.

Happy to help with this. You don't need to send pictures, this is old hat. We do this all the time.

We build remarkable!

Ron

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John in Pickering
Date/Time2/15/2020 at 3:42:11 PM

I just went into the basement to evaluate the area. I noted the following (my basement is unfinished).

1. the foundation walls are 7.5ft tall and approximately 10-12 inches thick.

2. There is one small area with a crack 1/2 way down the wall - no cracks anywhere else on the property

3. The first floor trusses sit essentially on top of a piece of wood that sits on the foundation.

So if we tear the whole house down, would we not have to re-pour concrete and connect it to the current foundation to make it higher?

Or can we tear the home down and then connect the first floor trusses 1.5-2ft higher on the first floor and then do the same thing on the second? floor?

Thanks everyone!

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Date/Time2/15/2020 at 3:58:12 PM

Your existing foundation will need to be raised to your desired height and then your first floor joists will sit on the top of a wood top plate, similar to your existing structure.

Your engineer will lay out any new structure, if needed.

Justin 647-884-3550

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John in Pickering
Date/Time2/15/2020 at 4:11:52 PM

Ok yes this is what I figured.

Now is it structurally possible to add and pour more concrete to the existing foundation? I assume you have to drill holes and add rebar to the existing foundation then box out and pour the concrete on top correct??

Has anyone here seen or done this before?

Also finally - what is the cost to just do a new foundation instead? Our current foundation is somwehree around 70ft by 40 feet.

Thank you!

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Date/Time2/15/2020 at 4:54:03 PM

From my understanding with your details I would suggest you ask a structural engineer to get your correct answer since he will know your local building requirements snow load! Existing Foundation load!

Concerning building it I would say it's very doable. All you need is engineered floor joists to increase the basement height and go right on top of the existing concrete Foundation walls since they are 10 to 12 in I don't think you have an issue whatsoever.

Again I just recommend you contact the structural engineer he can give you the hundred percent answer you're looking for.

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Date/Time2/15/2020 at 5:33:06 PM

Unfortunately in this trade there is lots of assumptions from contractors..

Some contractors like to assume everything is prefect when they answer general questions.. And that's why lots of project excess the budget heavily.

We have some contractor here saying it is OK to do so and they do it all the time.. And some even go to talk about other contractor ability..

In the prefect world it is possible to just say yes. In reality until contractor or engineer could see the site there is lots of unknown.. We seen houses that is missing almost half the soil under the foundation because of leaks that's been happening for a long time..

At the end of the day. Project like this is not a small undertaking and yes is possible but no you don't wanted to just asked on the forum to get an answer.. As one of the other contractor say, if foundation is sound and all then sure, but always check with your engineer instead of being sorry.

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Date/Time2/15/2020 at 5:45:39 PM

Hi,

I have read all the posts so far. And I must agree with Gene's post from Etobicoke.

There are many subtle variables thst might exist.

Get an engineer with experiance out there to go over the existing foundation in detail. In the past I have had engineers discover problems prior to our adding a second floor on a home in south Oakville.

All the best with your project.

James

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Date/Time2/15/2020 at 6:29:26 PM

This is might be doable but all based on a structure engineer calculations and how the numbers say but you might do not need to do anything with the foundation as long as the foot print will stay the same

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John in Pickering
Date/Time2/16/2020 at 1:06:41 AM

Yes Mike,

The only reason we want to tear down is because the house has a vinyl exterior and only 8 foot ceilings. We figured by the time we redo everything like brick or stone exterior, windows and doorsd, and gut the entire home it would be close to spending what it would cost to tear down .

We like the house shape and size and feel we can gain more with an extension in certain areas to make the home our own .

So if we could reuse the existing foundation, extend the home in other areas, and add around 1.5ft of height to the foundation walls without tearing them down we could build on top of this existing structure and save a bit in the process

I've tried to YouTube videos or search online to see if anyone has added height to existing foundation walls but I have not been able to find anything similar

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Date/Time2/16/2020 at 1:37:16 AM

To increase an existing foundation, rebar driller every 16 is required but an engineer will give you exact details and lay out all requirements.

With respect to Gene's post, the question asked was if it was possible. The answer is yes. There are structure issues that only an engineer can answer for you and start there.

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Date/Time2/17/2020 at 6:50:05 PM

In respect to Justin.

The initial question is would this "be easy to do or even possible, or is it better to just start from scratch and redo"

Well all I was saying is that until engineers look at it, it is hard to say if it could be done easily or not.. Or if it is more cost saving or not.

As of is it doable. Well we human did send people to the moon and build subway under an existing city so I am pretty sure is possible..

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Date/Time2/18/2020 at 4:44:17 PM

Hey John,

it is possible just have the existing walls checked by an experienced structural engineer, not a kid straight out of school because they were cheaper, not the area you want to reduce costs on this particular item. Also if you are adding a lot more to the height add up your height with the additional floors plus roof and make sure you are within your max height for zoning in your area for residential dwelling. no point hiring an engineer and doing all that work just to have the city deny your building because youre over your max height

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