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How much slope

by Mark Barden last modified May 26, 2006 12:32 PM
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How much slope

Posted by Mark Barden at May 22. 2006

Much of the literature says prefab homes can be built on a slope. Does anyone know if this is truly the case and if there are any parameters? I don't want to buy an expensive piece of land to be told that it is unfeasible to build. Anyone know any details? Thank you.

Re: How much slope

Posted by mjfree at May 23. 2006

For one, a steep lot should be much less expensive than a flat lot. So, in relative terms, steep lots should be inexpensive (though I know that the absolute magnitude is high).

The expense of the foundation is dependent on the prefab long dimension, lot slope (up, down, %), region of the world (wind, seismic, snow), etc.

I was looking at land in the SF bay area. For what it is worth, my RE agent told me that a foundation on a steep lot I was looking at was likely to be 100k greater than the flat lot. If the free market works properly, and there are enough professional spec. builders, then the lot price should dictate the cost to build, minus a little bit for risk. In other words, buying a sloped lot is more about availability than affordability since the delta cost of site work/foundation will likely wash out and savings you get for buying a sloped lot (except you may be able to save a little).

What area in specific are you looking?

Re: How much slope

Posted by Mark Barden at May 24. 2006

Thanks for this. We are looking in the Bay Area - North or Sounth Bay. It's more about a nice spot than the precise geography, so we are flexible.

There is land available, but little of it is flat and, obviously, one of the things that is nice about some of the Modern designs is they are built on the premise of flat land - courtyards, breezeways etc. Stacking on a slope may not be as practical or as aesthetically pleasing. Do you know of any pix of such a configuation?

And clearly, the issue here is getting a tight budget. It's hard to buy land with certain assumptions about cost to then find an extra $100,000 required for foundations. Doing all the homework on land before working with an architect is a daunting prospect unless one is an expert in such things. Too many unknowns and gambles.

Re: How much slope

Posted by mjfree at May 24. 2006

I would recommend having an architect (at a minimum) and an engineer on board before purchasing land. They will see things that effect cost to build that you cannot. A good architect will have an idea on build costs, including the foundation costs (pier and grade beam vs spread footing).

I remember how my wife and I (both structural engineers) thought we would build for substantially cheaper than buying a house. Our architect told us how much it would cost to build our design, and we didnt believe him. In the end, he hit the price almost spot-on. Luckily, we secured a loan for the amount the architect recommended, and not our optimistic outloook. At the end, we spent nearly every penny of our construction loan, and the only way we finished the project was a whole bunch of sweat equity.

In the bay area, you would be lucky to keep construction costs under 225.00/sqft. Add on top of that land and soft costs (permit, arch, eng, etc), to build the most modest new home in the bay area will cost ~700k.

You could try modern pre-fab, but all those prices are in the 200's/sqft.

Re: How much slope

Posted by mjfree at May 24. 2006

one more thing...

Prefab homes have different rules in the county building departments than do custom site built homes. Since prefab homes are covered under HUD rules, they get away with real sketchy foundations. My wife, a civil-structural engineer, was appalled at the cavalier attitude of 75% of pre-fab home installers towards foundation engineering. Most of these people want to sell you the home, and convince you that the engineer in nebraska will do your foundation analysis. This works, of course, because HUD exists to make it easier to get into houses = less regulations. Especially on a sloped site, I would not believe a word the prefab companies say about in-house engineering, unless they have somebody on staff with a California PE license that can do seismic analysis.

Re: How much slope

Posted by Mark Meyer at May 25. 2006

I need to clarify that ANY prefab home that will meet city code is not governed by HUD. Trailer homes are the only prefabs that are still covered by HUD. ANY prefab meant to be placed over a permanent foundation is International Residential Code compliant. This of course doesn't mean that the companies WON'T try to convince you that their staff engineer can design the foundation for you, because they more than likely WILL, but for a sloped lot in a seismic zone, I'd not hesitate to have a local structural engineer handle that part.


Re: How much slope

Posted by mjfree at May 26. 2006

Is this a new consideration for HUD?

I have to admit - I have not explored the process of installed a pre-fab past talking with local manufactured home companies. I was told that their homes fall within HUD requirements, and therefore we can get away with a simple foundation... I thought that I didnt want to get away with my house settling into the ground or falling off the foundation during an earthquate.

So, is it true that manufactured homes are covered by HUD and not prefab? Second, is there a difference between prefab and manufactured? In my mind, it is simply semantics, but if there is a clear difference, I would love to know. I know that pre-fab tends to be more custom, but other than that????

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