Expert views needed: Building an addition on piers in DC on a semi detached house - can it be done?

[Anonymous]

Hi DCUM! We were planning to build a 12x18 addition on piers on our semi-detached home. It currently has an 8x18 two level enclosed sleeping porch off the back which we know is not stable and needs to be torn down.

The 12x18 addition on piers would be two floors - one bedroom with simple bath (shower/toilet/sink - no tub) and an ikea kitchen on the first floor.

A family friend who is starting out in an established design build firm told us we would **have** to build the 12x18 addition on a crawl space and could not build it on piers.

We don't need to use the existing piers. New piers are fine. We were surprised by his statement.

Is this true? Is our friend correct or inexperienced?

We are in NW DC. For various reasons we do not want to do the crawlspace. Is there code or a city requirement that stipulates a 12x18 addition had to be over a concrete crawlspace?

[Anonymous]

I don’t know. I know you rarely see piers in DC. In fact I recently saw a house on piers and did a double take because I never saw that before. So I assume there may be a code issue.

[Anonymous]

Talk to an engineer.

[Anonymous]

thank you! Is there anyone out there who has done this? I've seen new piers on rebuilt sleeping porches in the same footprint. And i have seen new ones on ones that go beyond the footprint, but there is a larger gap (3-5 feet instead of 1-2) between the semi detached houses in that case. Not sure why.

[Anonymous]

You’re going to need an architect either way, so engage one with local experience and ask them.

[Anonymous]

I just did this. DC has a strong tree preservation law, you can't disturb the roots of a mature tree. In order to expand my footprint I had to build on piers. Helical piers to be specific. I've also talked to neighbors whose house was built on fill -- this is in Palisades/AU Park -- and in order to hit bearing soil they had to go down 40+ feet, they used helical piers as well because they don't get much more expensive the deeper you go.

The guy who installed my piers seemed to be quite busy.

Here's where your friend is half-right: DC has what they call a "prescriptive" building code. That means there are certain ways of doing things that are prescribed in the code, if you do it exactly that way you don't need an engineer. So the code will say that the prescriptive way is to build a foundation with footers that are X inches thick and Y inches wide and you're good. If you don't do it the prescribed way, you can hire an engineer and have him design a custom system for you.

Prescriptive is always going to be cheaper -- you don't need an engineer, and the contractors you work with are going to be familiar with the technique. With a custom design not only do you have to pay the cost of the engineer, but the contractors are taking on risk that they don't do it right, and they are going to pass that cost on to you in their pricing.

Where your friend is right is that a crawlspace is the prescriptive method. Piers are a custom method. But they are allowed and an experienced engineer should have no problem with them.

[Anonymous]

Anonymous wrote:Hi DCUM! We were planning to build a 12x18 addition on piers on our semi-detached home. It currently has an 8x18 two level enclosed sleeping porch off the back which we know is not stable and needs to be torn down.

The 12x18 addition on piers would be two floors - one bedroom with simple bath (shower/toilet/sink - no tub) and an ikea kitchen on the first floor.

A family friend who is starting out in an established design build firm told us we would **have** to build the 12x18 addition on a crawl space and could not build it on piers.

We don't need to use the existing piers. New piers are fine. We were surprised by his statement.

Is this true? Is our friend correct or inexperienced?

We are in NW DC. For various reasons we do not want to do the crawlspace. Is there code or a city requirement that stipulates a 12x18 addition had to be over a concrete crawlspace?

Ok, my curiosity had gotten the best of me. Are bodies buried there? Something else? Why are you insisting on piers and not going with the traditional/prescribed crawl space?

[Anonymous]

Anonymous wrote:

Anonymous wrote:Hi DCUM! We were planning to build a 12x18 addition on piers on our semi-detached home. It currently has an 8x18 two level enclosed sleeping porch off the back which we know is not stable and needs to be torn down.

The 12x18 addition on piers would be two floors - one bedroom with simple bath (shower/toilet/sink - no tub) and an ikea kitchen on the first floor.

A family friend who is starting out in an established design build firm told us we would **have** to build the 12x18 addition on a crawl space and could not build it on piers.

We don't need to use the existing piers. New piers are fine. We were surprised by his statement.

Is this true? Is our friend correct or inexperienced?

We are in NW DC. For various reasons we do not want to do the crawlspace. Is there code or a city requirement that stipulates a 12x18 addition had to be over a concrete crawlspace?

Ok, my curiosity had gotten the best of me. Are bodies buried there? Something else? Why are you insisting on piers and not going with the traditional/prescribed crawl space?

Not OP but crawl spaces don’t really bring a benefit. I feel piers give you more flexibility, I am not a big fan of CMU foundation and solid pour can easily be more than piers. Plus on an addition piers may easier logistically.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Hi DCUM! We were planning to build a 12x18 addition on piers on our semi-detached home. It currently has an 8x18 two level enclosed sleeping porch off the back which we know is not stable and needs to be torn down.

The 12x18 addition on piers would be two floors - one bedroom with simple bath (shower/toilet/sink - no tub) and an ikea kitchen on the first floor.

A family friend who is starting out in an established design build firm told us we would **have** to build the 12x18 addition on a crawl space and could not build it on piers.

We don't need to use the existing piers. New piers are fine. We were surprised by his statement.

Is this true? Is our friend correct or inexperienced?

We are in NW DC. For various reasons we do not want to do the crawlspace. Is there code or a city requirement that stipulates a 12x18 addition had to be over a concrete crawlspace?

Ok, my curiosity had gotten the best of me. Are bodies buried there? Something else? Why are you insisting on piers and not going with the traditional/prescribed crawl space?

Not OP but crawl spaces don’t really bring a benefit. I feel piers give you more flexibility, I am not a big fan of CMU foundation and solid pour can easily be more than piers. Plus on an addition piers may easier logistically.

I'm the PP from 9:46 who has done this. I don't see any advantage to piers over a conventional crawl space. The big disadvantage is you can't encapsulate it, you have to insulate and seal the floor and it's hard to do well. I had to have the insulation crew back four times before they'd done something I'd consider semi-proper.

[Anonymous]

The space will be harder to heat and cool if built on piers. In the winter you may risk pipes freezing.

[Anonymous]

Helical piers are SO expensive.

[Anonymous]

Anonymous wrote:Helical piers are SO expensive.

No once you factor in time and logistics. Plus you can typically use fewer than poured piers.

[Anonymous]

Anonymous wrote:The space will be harder to heat and cool if built on piers. In the winter you may risk pipes freezing.

My thought also, pay extra attention to insulation, those rooms will be tricky to keep comfortable in the cold.

[Anonymous]

Anonymous wrote:The space will be harder to heat and cool if built on piers. In the winter you may risk pipes freezing.

Read this article: https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

Almost no one knows how to insulate a crawl space properly, they usually end up as a moldy mess.

[Anonymous]

Anonymous wrote:

Anonymous wrote:The space will be harder to heat and cool if built on piers. In the winter you may risk pipes freezing.

Read this article: https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

Almost no one knows how to insulate a crawl space properly, they usually end up as a moldy mess.

This most contractors don’t know how to insulate at all, let alone properly do a crawl space.

We just built a house in a humid climate and the methods that subs wanted to use were laughable. Then when you would point out them what they were neglecting to address, they were shocked. So it wasn’t like they were trying to get away with shortcuts, they genuinely had no idea.