How to Choose the Right Greenhouse Size (Without Regret)

By Ryan, Planet Greenhouse

After foundation, sizing is the single most common question we walk buyers through on the phone. And the honest answer is that most people first-guess too small. They picture a couple of tomato plants and a potting bench, and then a year in they wish they'd gone one size up. So before you settle on a footprint, let's walk through what actually determines the right greenhouse size — with real numbers for the Janssens line and the mistakes we watch buyers avoid.

What actually decides the right greenhouse size

Three inputs decide it, in this order:

1. What you want to grow. Herbs and a few seedlings need very little. Tomatoes, peppers, and citrus in pots need real bench space, real headroom, and a proper ventilation plan. Year-round growing needs more space still — you're storing tender plants over winter and starting seeds in spring at the same time.
2. How much time you'll spend in it. A greenhouse you visit once a week and one you work in daily are different structures. If two people are ever in it together — very common on install day and again at spring transplant — you need room for two people, a cart, and benches on both walls.
3. Site constraints. Setbacks, slopes, permits, and the foundation budget all scale with footprint. A 10'x15' greenhouse and a 12'x20' greenhouse aren't just different in price — they're different in concrete, freight, and often permit tier. (More on that below, and in our greenhouse permit guide.)

The mistake is starting with a price and working down to a size. Start with what you want the greenhouse to do, then match the model to that.

The Janssens sizes at a glance

Janssens of Belgium builds Royal Victorian, Modern, Cathedral, Royal Orangerie, and Gigant lines — manufactured in Lier, Belgium and imported to North America by Exaco. Here are the real footprints and where each one lands.

A Janssens Royal Victorian has a 45° roof pitch, which gives real headroom at the ridge — the smallest VI23 still has room to stand fully upright inside, and tall crops like indeterminate tomatoes or potted citrus fit comfortably even in the mid-size models.

How much growing space you actually get

Square footage on a spec sheet isn't growing space. Growing space is bench area plus floor area minus the walkway. A quick worked example on the Royal Victorian VI34 (10'2" × 15', ~153 sq ft):

• Run 2-foot-deep perimeter benches along both long walls and the wall opposite the door.
• Bench length: 15 ft + 15 ft + 10.2 ft = ~40 linear feet
• Bench area: 40 ft × 2 ft = ~80 sq ft of usable bench
• Center walkway: about 30" wide down the middle
• Floor space under and around the benches for larger pots and tender trees

That's a very generous working greenhouse for one gardener, tight for two. If you want to plant directly into the ground in the middle (a common ask with tomatoes and cucumbers), the VI36 or VI46 opens that up without giving up the perimeter benches.

Rule of thumb: minimum walkway width is 30 inches. If you'll ever wheel a cart, a wheelbarrow, or a bag of soil through, go 36–42 inches — which pushes you toward the VI46 or wider.

Sizing by what you'll actually grow

The fastest way to land on a size is to match it to the job. Here's how we sort it on a call:

The pattern we see most often: buyers first ask about a VI23, and by the end of the call they've landed on a VI34 or VI36. Not because we upsold them — because once you count benches, walkway, seed-starting shelves, and one tender tree in a pot, a 79 sq ft greenhouse fills up fast.

The three sizing regrets we hear most

The three things buyers tell us they'd change if they were choosing again:

1. "I wish I'd gone one size up." By far the most common. Fixing it later means a new foundation, new freight, and often a permit re-file — real money. The upgrade from VI34 to VI36 costs a fraction of that on the first order.
2. "I should have gone with a double door." A single sliding door works, but a double sliding door (standard on the VI36 and VI46) is the difference between wheeling a cart through and turning it sideways. If you'll ever move large pots, potting soil, or a tomato harvest through the door, spec the double. Our sliding vs hinged door guide walks through the trade-off.
3. "I didn't leave enough exterior clearance." Every Janssens greenhouse needs about 30 inches of clearance around the perimeter so you can set a ladder, service glazing, and clean gutters. Buyers who tuck the greenhouse tight against a fence or a garden bed regret it the first time a pane needs replacing.

None of these are dealbreakers you can't work around. They're just the exact three things a five-minute phone call would have prevented.

The site constraints people forget

Bigger greenhouse means bigger everything downstream. Before you settle on a size, count the site costs:

• Foundation scales with footprint. An 8-inch concrete strip foundation on a VI34 vs a VI46 is a meaningful difference in concrete, rebar, and labor. Full breakdown in our foundation guide.
• Permits often tier by square footage. Many US jurisdictions exempt structures under 120–200 sq ft from permitting; go over that line and you're in a different process. Confirm locally, and see our permit guide.
• Setbacks matter. A larger footprint may not fit within your side-yard or rear-yard setback — check before you fall in love with the VI46.
• Freight and install labor both scale with size. We publish real installed numbers in our greenhouse cost breakdown.

None of these should push you to a smaller greenhouse than you'll actually enjoy — they're just the line items to price out with the model, not after.

So what should you actually get?

Here's the short version of how we sort a sizing call:

• One gardener, herbs and seedlings, tight budget → VI23
• Family veg, tomatoes and peppers, want room to work → VI34 (the most common answer, and rarely regretted)
• Year-round growing, overwintering plants, two people in it regularly → VI36 or VI46
• Heavy snow, working grower, portal-frame construction → Cathedral
• Architectural moment near the house → Royal Orangerie
• Custom project, multi-zone, entertaining, or wedding-scale → Gigant (custom)

Every one of these ships as heavy-duty powder-coated aluminum, 4mm tempered safety glass (or 10mm polycarbonate, or hybrid), manufactured in Lier, Belgium, and backed by a 15-year warranty. The frame, glazing, and warranty are the same across the Royal Victorian line — you're choosing footprint and door/vent count, not build quality.

Questions people ask about greenhouse size

What is a good size for a hobby greenhouse? For a family gardener growing tomatoes, peppers, and seedlings, we most often recommend the Royal Victorian VI34 (10'2" × 15', ~153 sq ft). It gives you benches on three walls, a real walkway down the middle, and enough headroom for tall crops. If you'll be in it year-round or with two people, the VI36 or VI46 is the safer pick.

How big of a greenhouse do I need for a family of four? A family growing a meaningful share of their vegetables typically wants ~150–250 sq ft — the VI34, VI36, or VI46. That's enough for perimeter benches, in-ground planting in the middle, and seed-starting shelves without crowding the walkway. Below ~150 sq ft you'll run out of space in the second year.

What's the biggest Janssens greenhouse? The Janssens Gigant is the largest, with custom sizing up to 20 feet wide by 60 feet long — roughly 1,200 sq ft. It's used for multi-zone growing, entertaining, and even as a wedding venue. See our custom Gigant builds guide for full detail.

Do I need a permit for a large greenhouse? Often yes, though it varies by jurisdiction. Many US municipalities exempt structures under 120–200 sq ft from permitting; anything larger typically requires a permit. Our greenhouse permit guide walks through what to expect and what your building department will ask for.

Can I add on to a greenhouse later if I go too small? Janssens Royal Victorian greenhouses aren't modular — you don't bolt on another bay after the fact. If you outgrow it, you're either selling the smaller unit or building a second structure. That's the practical reason we push buyers to go one size up on the first order.

How much clearance do I need around a greenhouse? Leave about 30 inches of clearance around the entire perimeter. That's enough to set a ladder for glazing service, clean gutters, and replace a pane without dismantling anything. Tucking a greenhouse tight against a fence or a raised bed is one of the most common regrets we hear.

Still not sure? Let's talk through your site

Sizing is one of those decisions that's obvious once someone walks you through your climate, your growing plans, and your site — and genuinely hard to get right from a spec sheet. Most of the buyers we help land on a size within about fifteen minutes on the phone.

Tell us what you want to grow, how many people will be in it, and roughly where you're placing it, and we'll tell you honestly which Janssens size fits. Give us a call or reach out anytime — no pressure, no invented urgency, just a straight answer from someone who does this every day.