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Looking for answers from people building foundations near (not in) the 100 year floodplain. Does this trigger a floodplain build permitting process? Am I basically pigeonholed into a monolithic pour to avoid this? No one working on my foundation (excavators, concrete company etc) seems to know the answer or they think I should just "not worry about it".

My question is 100% a FEMA rules / what is allowed question. Not at ALL interested in whether I "should" or not. To dissuade your fears: This is a field in Montana near a creek that hasn't flooded the site in the last 100 years. I have some control over the flow in the creek. There are hundreds of houses around me, (many lower) which have never flooded (in 100 years). Even if this is all a terrible idea, I simply think it's worth the risk.

I DO care about the county or FEMA saying I did something wrong / making me change it. And, I DO care about a mortgage company forcing flood insurance (showing I am somehow in a floodplain) - pretty sure that won't happen however.

A picture says a thousand words:

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Floor joists and the entire home and all electrical, 100% of everything is above the flood plain elevation.

I must: Apply for a permit for any modification or even moving a rock inside the 100 year floodplain. Does this count since I'm digging beneath it? The county did not specify and there no building permits in this county. They were adamant on proving my site is outside the 100 year floodplain (unless flood plain permitted).

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    Given "My question is 100% a FEMA rules / what is allowed question" then ask FEMA for clarification. – Solar Mike Jun 30 at 17:34
  • @SolarMike I wouldn't ask FEMA the answer to 1 + 1 even if their rule book said " answer = 3". Like most enforcement agencies, they generally aren't familiar with their own rules until they want to enforce them after the fact through some auditing process. – maplemale Jun 30 at 17:36
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    @maplemale "Still, this question stands on its own and is different than the one you are referencing." Agreed, i was not suggesting otherwise. – Alaska Man Jun 30 at 17:50
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    Are you sure this property is actually in a flood plain when measured via altitude? The USGS just arbitrarily drew lines called "flood plain" for a specific horizontal distance from a body of water that could flood. This is a very poor approximation but it was cheap and easy. For example my property was originally designated flood plain but it is, in fact, 44' above the 100 year flood line. All it takes is a survey to show that it's not actually flood plain. – jwh20 Jun 30 at 18:05
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    Our crawlspace floor (soil) is below the 8' base flood plain elevation, probably at 5'. But the first floor is above that 8' BFE. So no flood insurance required. – SteveSh Jul 1 at 0:17
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The flood plain issues I had to deal with is more about the openings in the foundation to allow water to exit. As others have said have a Survey, my current home was in the flood plain until the survey , huge insurance jump because the lender required flood plain insurance until the survey. I am planning on an additional 3 car garage the county is the one that issue the permits, not fema.

If the county asks show them the structure is not in the flood plain, when you get your permit build to your plot plan and you will be fine. I have a friend that built a house on poles to put the structure above the flood plain his footings for the poles were well below the 8’ or more “level identified as the 100 year level” the county only required engineering stamps on the plans, just like roof trusses require in most locations. He has never had to have flood insurance.

Don’t bring up issues that you don’t want to deal with because as you said they go to the ultra safe side because they don’t want to be wrong. After living through several 500+ year floods he started enclosing the lower section I have not talked to him but last time I was in Corvallis the entire lower 1/2 was enclosed.

Monolithic pours are not the only way to go but you are not in the flood plane so get your permit. Make sure you plan for drainage like a French drain under the low side with a heavy footing above that for insurance that it will drain this doesn’t have to be on the plans. So submit your plot plan to the county and pay the fee. If they ask , show the survey. When approved start building and enjoy your home.

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  • Thanks very much for this answer! Corvallis? As in Ravalli County MT? Nice - same county i'm building in. Actually, this county requires zero build permits. It's state permits only: Electric / plumbing – maplemale Jun 30 at 19:59
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    Corvallis in Oregon , wow you are lucky we have permits for everything here, well, septic, plot plan, house , electrical if the property crosses a creak that’s has fish just add another 40-80k for the required bridge, but it’s a nice area. – Ed Beal Jun 30 at 20:06
  • Ed Beal what about surface swales or vertical drains to underground piping on the high side to intercept surface water moving downhill and piling up against the foundation? – Jim Stewart Jul 1 at 0:40
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    @JimStewart I'm 90% on the flat. I used to have a house in Newport, OR not far from Ed Beal. It was easy there because it was hilly and draining strategy was obvious. Over here, the highest spot on the whole 56 acres is where i'm building and it's only a couple of feet higher than the rest. Not sure how to do drainage properly here and will have to do some reading / get some advice. Most builders here simply don't do it unless you're on a hill to keep runoff away. – maplemale Jul 1 at 15:20
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    @Ed Beal - Yes. Down here on the Delmarva Peninsula and along the coast, if you're building in the flood zone (simplistically, 8' above the mean high tide mark), you're required to install flood vents so that if any flood waters get in, it can escape without exerting undue outward pressure on the walls. – SteveSh Jul 1 at 23:48

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