Ten feet. Why worry a little when you can worry a lot??

Why isn’t 0 an option? This makes no sense at all if you’re not in a flood plain. Might as well load up 4 extra tires on rims into the cargo area and not trust the donut spare to get you to a tire store.

How close are you to a flood plain? Has the area ever flooded? Our house isn't in a flood plain but it did flood once (in 1979) and there are flood plains a block away with only very subtle changes in elevation (a few inches) between our house and the flood plain. In a situation like ours, I don't think it would be unreasonable to consider a higher foundation. But if your lot is three feet higher than the nearest floodplain, an elevated foundation might be overkill.

The default foundation is supposedly 6 inches.

I don't trust the floodplain maps because the TX State Capitol building flooded this year during the storms even though it's not technically in a floodplain either.

I'm less than half a mile of mostly flat land from the river, which is officially a floodplain.

As sure as you worry about this...................It will be the rarer tornado that takes the house. Just believe the map. and plan no control over climate change. Climate has been changing for thousands of years. We are mere specks in geological time. .......as are weather records.

IMO not the place to get the answer talk to the powers that be in your area and a good architect .

The Texas State Capitol is on quite a hill. Any water damage it got was not from flood plain rising. Your question is not answerable. I've seen houses in Texas that were not in mapped flood zones have 3 feet of water in them. You'll have to analyze your lot and see if there's a potential issue.

I think that it's a fair concern, but I'd solve it more by finding the best elevation that I could in the surrounding area first and then maybe building up a lesser amount. Having your final floor elevation 2' higher than all your neighbors will just be weird.

Seek higher ground and then do some light grading and smaller raised foundation.

Neighbor on the right has a pier and beam foundation so they're way up high already (4-5 step staircase up to their porch); neighbor on the left has a standard slab on grade foundation. I'm not super concerned about being out of step with the neighbors given the situation. Perhaps I could split the difference?

I'm on an urban lot with lots of trees, so I'm constrained in terms of site location. There is exactly one reasonable spot to place the house and the elevation there isn't that different from the rest of the lot.

I really appreciate everyone who answered the question thoughtfully rather than with snark.

Raising a slab on grade is very expensive and can be problematic depending on the design, installation, and materials. So let's say you bite the bullet and pay for a 4" higher slab. Would that really alleviate your fears?

Go with a raised floor and crawlspace.

Hm. This helps frame the issue somewhat because we had originally selected a slab on grade foundation due to concerns about pests, mold, and insulation with pier and beam construction.

I'll have to make a tradeoff between those fears (more common, to the best of my understanding) and my fears of flooding (Black Swan-type event).

If you have an elevation certificate, you can find out how many inches you are above the 500 year flood plain. If you are already two feet above the 500 year flood plain, I would go with the slab. If you aren't, then I'd go with pier and beam, which also has the benefit of being easier to fix if the foundation settles. Your architect and engineer should be able to design a properly ventilated and insulated crawlspace. I don't think you are being overly concerned about flooding; it happened to me, and it's not fun.

Pier and beam have a number of advantages besides keeping you up out of a flood. For instance, if you have a plumbing leak and your pipes run underneath the house, the damage and cost to fix will be minimal compared to what you would face if your water lines ran through your attic or were embedded in a slab. If designed right, mold, pests, and insulation don't have to be an issue. After comparing my experiences with a house on piers to those of friends with houses on slabs, I would choose the piers any day.

Keep in mind there are areas in Austin that a crawl space is impractical due to very shallow limestone.

Are there any bodies of water nearby - any rivers or streams? If you aren't in proximity to water that can cause a flood problem you may not have to be concerned.

I’m in Savannah Ga which has pretty good elevation (the oldest part of the city is on a 40’ natural rise) but even so street flooding is incredibly common- in a summer storm my neighbors can have water up to their front doors. Standard here for new construction is at least a couple feet above grade- usually a raised slab on compacted fill. I think most styles of houses look better slightly raised anyhow.

You should reach out to a geologist in your area and get an informed answer. They would be able to look at the outdated maps and consider a long term recommendation for you.