Greenhouse project planning

I’m in the earliest planning stages for my summer/fall goal of building a greenhouse before next winter, and I’ve been mightily struggling with deciding which of three possible locations I should choose. So, I’m crowdsourcing an answer…

First off, it’s not going to be too large, my goal is to have it be just large enough for one or two avocado “frankentrees,” grafted with all the cold-hardiest varieties I can get my hands on, as well as ~15-20 small seedlings in 5-gal pots (I’ll give each seedling 1 winter in the greenhouse before putting it in the ground). I’m thinking minimum size is ~150 sq ft, but maybe slightly larger depending on site that’s chosen.

I’ll start with a photo of my back yard (this is an old photo from before I bought the house, I’ve already replaced the rotting deck with a stone patio):


Option A: North-side porch replacement

Our porch is pretty poorly constructed, with moldy/rotting framing, plywood floors beginning to rot (mushrooms growing on it), and plastic panel roof that flaps in the wind and leaks badly. But, it has a nice cinder block foundation wall, so we’re planning at some point in the near future to remove everything down to that wall, and build a new porch or similar space.

The main downside for converting this to a greenhouse space is that it’s on the wrong side of the house (north), though the western end does get good afternoon sun (during sunny seasons at least). Supplemental lighting would be an absolute necessity. It also replaces a space that’s useful as a porch.

Size: 8’ x 22’

Option B: Terraced garden bed away from house

This is the smallest of the three spaces (6’ x 22’), but also the sunniest, and would interfere the least with current yard usage, other than losing one of our few vegetable garden beds. Other cons include not being sure about the structural integrity of the drystone walls above and below the terrace, so might need to improve/replace those walls before digging the foundation. Here’s another view of the garden bed:

Option C: Lawn against SW corner of house

The final choice is a level area adjacent to the SW corner of the house. The total area available here is 18’ x 20’, but I’d likely use less than that.

Major pros include relatively good sun (might need to trim down those cedars), though the neighbor’s house does shade it a bit when the sun angle is lower in winter. It’s also very level, adjacent to an outdoor power outlet, and I might be able to use the dryer vent as a source of waste heat? Major con is it would block the main path around that side of the house, and take up the only bit of sunny lawn on the same level as the patio. This is also one of the best areas for planting out the seedlings from the greenhouse, and I could probably squeeze quite a few into this space if the greenhouse is located elsewhere.

So what’s the verdict?

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Why was D: the area west of the stone patio not an option. Would seem to be roomy and sunny. How much digging there would be required for leveling?

Dryer vent would be an insignificant source of heat unless you are running a laundry service.


The way my kids go through clothes, sometimes it feels like I am! But I was only half-serious about that part.

That area is slanted, rocky, and I’ve already put a small pond there, which means the only available space is further down on the lowest grassy level. It was requested that I leave that grassy area between the large California bay laurel (or “Oregon myrtle” if you prefer :joy:) and patio as a kids’ play area, but I could maybe still negotiate a greenhouse there if you think it’s the best spot. Here’s a photo looking down from the porch that way:

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Could you do it so that you use the C space with the back wall being the house (similar to the side porch option)? If you would still get reasonable light I think that could be a good option. I believe lean-to types are easier to heat since you aren’t losing heat through the house side. I don’t have a greenhouse but have always dreamed of having one that was attached to the house with a door from the house directly into the greenhouse. That way I could just walk straight into it when it is cold outside and enjoy a cup of coffee sitting in the sun with my plants. And on some days when he heats up in the sun you can keep the door open to bring the free extra heat into the house.


I think that would be doable, though sadly no door to connect them, but at least there are three windows that could open into the greenhouse (basement laundry room, my office, and that small high window is in the shower stall, might be nice to open it and steam the greenhouse with every shower!).

edit: one downside of that is I would probably need to replace the outside window frames, and likely the old cedar shingles (original to the 1951 house) along that section of the wall, as I’m guessing the heat and humidity of the inside of a greenhouse would cause some acceleration of their already old/dry-rotted state. But that’s relatively minor compared to the scale of the project, and would be necessary for the porch option too.

Maybe get rid of the big tree at the bottom of the photo and put it down there away from the house. Or put it east of the tree.

The narrow bed and stone walls needs total rebuild if put there. Six ft is too narrow.

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I did broach the subject of removing the Umbellularia californica with the spouse when we first bought the house, but there was an absolute veto. We did get a notice from the power company that they will be trimming it this spring, though, so depending on how much they lop off the top it might be a topic to bring up again then.

That would be the time to turn that window into a glass door into the greenhouse, lol. By the way, my ideal greenhouse is probably different than a big “growers” greenhouse. I will just always remember walking into the conservatory in the Mark Twain House in Hartford, CT in the middle of winter when it was snowy outside and thinking how awesome it was to have a greenhouse part of your actual house. You can see the conservatory on the house’s site describing the rooms. Right above the conservatory, you can see in the picture of the Library and how you just walk right from the library into the conservatory. Of course for warmer zones a door would be good to keep out the summer heat and the AC in.


I say option C-- just have two doors, one on the east side facing your raised bed alley and one on the north side. Lean to would be easiest for rain run off.

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I’d lean towards C since a shed-type structure against a house is a neat greenhouse concept. A would be ideal if it was south but since you’re mainly interested in winter growing it’s tough

mainly I just want to commiserate over how poor light access is in most established neighborhoods in seattle/portland/etc. since shade trees grow so well and the sun is at a relatively low angle all year

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oh yeah, diffusion-type greenhouse fabric does make “A” almost workable since it lets you kinda redirect light around corners. it wouldn’t be ideal but you would still get some sun in there if it was on both side walls and the roof. in summer it would be plenty of sun. I think I’d prefer a higher diffusion fabric regardless of where you put it. this makes seed starting harder since they don’t get direct sun but you can use a small grow lamp for that

Amen to that. At solar noon on the winter solstice, the shadow of my south fence almost reaches the north fence. The shadow of those three tall cedars on the south fence easily reaches across the entire yard then, though a little less-so already by now.

One of the first decisions you may have to make is, are you doing this ‘by the books’, or on a ‘better to ask forgiveness than permission’ basis? Because this may affect your options. The manner of construction likely will as well.

If by the book, and depending on how web savvy you are, I would suggest doing some basic zoning research first, at the Seattle GIS site. Put your address in the top left search bar, and try to find out what the zoning of your property is. After you know your zoning, look up that zoning, perhapsat this site. This is potentially important because you most likely have setbacks/sideyards on all sides of your property that prevent you from building up to the property lines. And on the south side, that setback is potentially the same as the south wall of your house. So your available space might be more like 14x18’ there, rather than 22x18’, if you have an 8’ setback, which it appears you might (5’ would be more typical). Your rear yard setback will probably be much deeper, so probably a good thing you haven’t any plans to build there. Your north porch may violate those setbacks, but be grandfathered in. Or, as an open porch, it does not violate them. But once you enclose it, you might be in violation. Grandfathering goes right out the window the moment you change the use or dimensions of the space.

If you can navigate those sites, it may give you some insight into what could happen if you try and do it the ‘proper’ way with permits. If those sites are too tricky, you should call the planning department and ask for them to tell you what your limitations will be. Beware, that greenhouse may not be a ‘permitted’ use on your property, unless under a certain square footage.

All that said, if you’re going to ‘wildcat’ it, then never mind. But just be aware that if you build something that violates zoning, and officials somehow find out about it and realize it is not grandfathered, they can make you tear it out.

Red tape aside, I would agree with the site C recommendation, and just say that if you’re going to do proper foundations, you could extend into area B as well. Especially if you have a hefty setback on that side. Such a setback would not be the end of the world - you could move your area B planting beds into that setback, thus making extending the greenhouse into area B more agreeable?

Also, do not underestimate the problems humidity and moisture can cause at the interface with the house on site C. I would go so far as to recommend possibly having a small semi-enclosed breezeway between them, that will keep the humidity from reaching the house (and also maintain a short circulation route all the way around the house).


Thanks for all the information!

I had looked into it briefly a few weeks ago (i.e., looked at the info packets from the zoning office), and it’s a 5 ft setback required on the sides. I believe based on our survey that the porch is actually conforming, or at worst a few inches off, as there’s a cinder block wall along the property line, which I believe is entirely on our side, and the edge of the porch foundation is about 4’ from that wall.

On the “C” side, I measured from about 3’ off the property line, so you’re right that with a setback it would be more like 18’ x 18’.

Here’s the general rule for our size lot, from the zoning office info packet, though I think we lose more in the back than this because there’s an alleyway right-of-way (starts roughly at the back fence, but that probably encroaches a bit):

Ah, you’ve already done some due diligence, very good! Did the packet address accessory structures? If not you may still want to talk to someone about permitted accessory uses and structures. You’ll want find out A) if greenhouses are an allowed accessory use, and B) what the square footage and height limits are. Often it’s easier to just talk to the zoning people, I find. Non-commercial greenhouses may be a blind spot in the code, if you’re lucky. Or maybe Seattle is progressive enough to have allowed them.

Edit: This blog post claims that they treat greenhouses like sheds, which would be good as sheds are always an allowed accessory structure. He also claims that sheds are allowed in your rear yard setback, as well, which they commonly would be.

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Here’s the pamphlet I got that diagram from:

Edit: just read through it more closely and here’s a regulation referenced in the chart at the end that I’m going to need to read! I get along very well with the neighbor to the south, I’m sure we could get them to agree to this

In a shocking turn of events, my wife has suggested demolishing the old (wood-frame, dirt-floor) detached garage that we just use for junk storage, and replacing it with a greenhouse. :heart_eyes:


That sounds great :+1: Good luck in building it!


Had a demolition guy come by today and he says the concrete foundation wall looks very sturdy so he thinks I should keep that and reuse it, both because removing it would add a lot to his price quote and because it should work well for a greenhouse, too.

That would be a pretty large greenhouse! 16’ x 20’, give or take a few inches. This project is moving from “drain the savings” into “rack up some debt” territory… :grimacing:

Here’s a photo of the garage-to-be-demolished, since I just realized no photos of it were included above:

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Just a idea.
What if you striped all the roofing and siding off that building and recovered it with clear green house materials.
Polycarbonate panels for roof polyethylene on walls.
May move and double up the 2x framing.
So like if it’s 16” centers on the framing , move every other one and double them to 32” centers .
Add some diagonal bracing ?
So , essentially repurpose what you already have ?

Just a thought