Every so often someone calls with a bigger idea than a catalog model. A greenhouse large enough to walk rows in. A glass garden room off a newly built home. A pavilion. In one case, a structure big enough to host a wedding. When a standard footprint won't do it, this is the conversation — custom Janssens builds, and the model that anchors them, the Gigant.
Here's what's actually possible, and why a build like this always starts with a call rather than a cart.
How big can a Janssens greenhouse get?
Bigger than most people imagine. The Gigant is the largest model Janssens makes, and it can be custom-sized up to roughly 20 feet by 60 feet. That's not a hobby greenhouse anymore — that's an architectural structure. At the top of that range, people use it as a genuine event space; we've seen the Gigant serve as a wedding venue.
Between the standard lineup and the Gigant, there's a lot of middle ground:
- The Cathedral — heavy-duty portal-frame construction, stocked around 15' × 19'10" — for buyers who want serious size and strength without going fully custom.
- The Royal Orangerie — a premium, architectural look at roughly 12'10" deep × 15'7" wide — for buyers who want the structure to be a centerpiece.
- Custom configurations of the Royal Victorian and Modern lines for footprints and layouts that the standard sizes don't quite hit.
If you're not sure which of these fits your vision, that's normal — the line between "big standard model" and "custom build" is exactly the kind of thing we sort out on the phone.
Who a custom build is really for
Two kinds of buyers land here.
The first is the serious grower who has outgrown a standard greenhouse and wants room to work — wide aisles, tall staging, space for citrus or a real propagation setup that runs all year.
The second is the custom-project owner, often someone finishing a newly built home, who wants the structure to be as much architecture as greenhouse — a glass garden room, a pavilion, a living space that happens to grow plants. If that's you, the Gigant and the Orangerie are where the conversation usually starts.
Either way, you're not buying off a shelf. You're scoping a structure to a site and a purpose — which is why the details below matter before anything gets ordered.
A few things people get wrong about the Gigant
Because these builds are custom, a couple of specifics catch buyers off guard. Let me get ahead of them:
- The Gigant comes standard with hinged doors, not sliding. Hinged doors suit a structure at this scale and open fully — but they're a different install from the sliding doors on smaller models. If you want to understand the trade-off, our sliding vs. hinged doors guide covers it.
- Vent counts and configuration depend on the size you build. A custom structure isn't a fixed parts list — the number of roof vents, doors, and windows scales with the footprint you choose. Don't anchor on a single spec sheet number; we'll confirm the exact configuration for your size when we scope it.
- Sizing is a conversation, not a dropdown. Because the Gigant is built to your dimensions, the right starting point is telling us the space you have and what you want to do in it — then we work backward to a buildable size.
I'd rather flag these now than have you discover them mid-order. Getting a big custom build right is mostly about nailing the details up front.
The foundation and install are a real project
The bigger the greenhouse, the more the foundation stops being a weekend pour and becomes engineered work. On the Cathedral, for instance, the portal frames and their corner anchors must be embedded in concrete — that's not optional, and your contractor needs to know it before they form anything. A Gigant at event scale is a structure you plan with a masonry contractor and, often, an installer who has walked your site.
That's a good thing at this size and price. You want it engineered right, not improvised. Our foundation guide covers the fundamentals, and if you'd rather not source a contractor cold, we can point you toward installers in our network who handle qualified prep and assembly. On doors, glazing, and accessories, it's almost always best to order everything with the greenhouse — freight-shipping large components separately later tends to get expensive.
Why this starts with a call
You can't really configure a wedding-scale glass structure through an online checkout, and you shouldn't have to. A custom build has too many decisions that depend on your actual site — dimensions, door placement, glazing for your climate, how the foundation gets engineered — to guess at from a product page.
So this is the one purchase where the phone genuinely is the starting line. We'll talk through the space you have, what you want to grow or host, and what it realistically takes to build it — no invented price, no pressure, just a real scope.
Every Janssens greenhouse, custom or standard, is backed by a 15-year warranty and built in Lier, Belgium — engineered to stand for decades, at any size.
Got a big idea — a grower's greenhouse, a garden room, or something at Gigant scale? Call us and let's scope it or send your dimensions and what you have in mind. Custom builds are some of our favorite projects, and the sooner we start talking, the better we can get it right.