Not a sexy topic but a few people have asked “Why not a full basement or just a slab foundation?” So I’ll lay out my thought process for choosing a half-height foundation/crawlspace.
It sort of happened by accident and evolution. I did not include in the brief to the architect a request for a specific type of foundation for the house. It has mostly come about as Matt, my builder, and I worked through space constraints on the property and budget. Cost is an absolute consideration because there is no one who would loan on this project so many of my choices were based on whether they fit in the amount of cash I have available.
Space under the house can be used for a few purposes: living, storage, mechanical, or none (i.e. slab). For me, the half-height turned out to be the best cost/benefit choice for those purposes.
A slab was the original idea – it’s the fastest and least expensive to excavate, form, pour, and finish. However, there was more to consider than speed and cost.
The greenhouse is on an almost perfect East/West axis which means there should not be any direct sun on the north side of the house meaning it’s not very useful for growing food or ornamental plants. So very early in the design process we tried to land on the highest value use of that space which then informed how wide the space between the house and the greenhouse’s north wall would be.
The decision for the lap pool being the highest value use of that space is because it provides multiple benefits – exercise, aesthetics, a cool-sink for the air-to-ground heat exchange system, and a source of irrigation water if the rainwater tanks run dry. Since the lap pool needed a concrete sidewall that became the number 1 reason for a 4′ concrete foundation wall for the north side of the house.
The requirement was to have all living space on one level that is wheelchair accessible. Therefore, I didn’t need or want a full-height foundation for additional living space. The house is sized for one person or a couple; I did not want guest space in the house as I’m keeping the trailer I’ve been living in as overnight guest space.
There are already 2 cabins on the property that I’ll use one for an office/craft room/YouTube studio and the other for a work room/shop/garden shed. These will provide a place for stuff that would be ‘stored’ in a garage or a craft/office room. But I want these things available for frequent use so it would not be convenient to have them in a storage room.
Also, I’ve purposefully reduced my possessions over the last few moves which means I don’t have much of the seasonal stuff that people store in attics, basements, or storage units. I like to be intentional with my possessions and do not want the temptation of too much storage space available that calls to be filled.
There is a small storage area planned for the carport that will be built after the house is finished. It is sized for a small freezer and few other items it would be inconvenient to have in the crawlspace. So I really didn’t need storage under the house. That said, once we’d settled on the half-height foundation/space under the house I reduced the size of the storage area in the carport thereby saving some money having less enclosed square footage and under a roof.
This is the reason that I, ultimately, decided on the half-height foundation for the entire house and not just the north wall – so that some of the mechanical equipment can go down there instead of either outside the greenhouse (where it would need protection from the elements) or inside the greenhouse taking up valuable real estate. So far the water filter is down there, the air pumps to circulate the pond/pool water will be down there, pipes for waste and exhaust, and not sure what else may end up down there.
Half-height versus full-height
Once the decision was made to have the entire house on at least a 4′ foundation, the choice to not make it a full height foundation was purely cost. If I were able to finance the construction this may not have been as black and white because I would have taken into consideration resale value of storage or living space. However, that is not the case so I’m making decisions based solely on my preferences and spending priorities.
My property is about half way up a slope that runs from the shoreline up to about 690 feet above sea level. Both my builder and the excavation contractor live on the island, have worked on several properties on the island, and had a pretty good idea what we’d find once they started digging. We expected to find (and did find) water running down hill through my property that would need to be diverted with foundation drains. (See YouTube episode 3). But I’m well above water level so we did not have to take water level into consideration for foundation depth.
First consideration was the cost to excavate for 8′ walls. We ran into rock in the northeast corner that would have added thousands of dollars and maybe a day or two to break out. Since we were able to get to within 6″ of desired depth without breaking the rock we decided to raise the final grade 6″ to compensate. (See YouTube episode 2)
Second consideration was the cost of concrete material and labor. Both are more than double what I paid for concrete at my last house. The bill for the retaining walls, house, and pool foundations could buy a nice vehicle. (See YouTube episode 5). Given that I didn’t need the extra space, the added cost to get a full-height foundation was not justified when weighed against what I’d have had to give up elsewhere.