Anna Persson

Everyone says buy bigger, and they're mostly right. But a too-big greenhouse costs more to heat every winter. Here's where the line is.

Quick answer: Buy one size larger than you think you need, then stop. Nearly every greenhouse owner fills the space faster than expected and wishes they had gone bigger, so an 8x10 or 8x12 is a safer starting point than a 6x8 for a serious gardener. But bigger is not free: a larger greenhouse costs more to heat every winter and is harder to keep from overheating in summer, so going two or three sizes up 'to be safe' can leave you paying to heat empty air. Size to your real use and your climate, add one step for growth, and match the heating budget to the volume you chose.

The kit is $8,000. The project is $12,000 to $18,000 once the base, anchoring, and auto vents are in. The line items nobody quotes.

Quick answer: A serious backyard greenhouse is an $8,000 structure that becomes a $12,000 to $18,000 project once you add the base, site prep, anchoring, automatic vents, and running power and water to it. The base alone runs $500 to $5,000 and is the single most under-budgeted line. A mid-market polycarbonate kit is a different budget entirely, roughly $1,500 to $6,000 for the kit plus a $500 to $2,000 base you still cannot skip. Price the whole project before you fall for a catalog photo, because the base, the anchoring, and the ventilation are the parts that decide whether it lasts.

The mid-market default, reviewed straight. Twin-wall to 10mm, 5 to 10 year warranty, and exactly where it falls short of a glasshouse.

Quick answer: Palram/Canopia is the mid-market greenhouse default, and for most step-up buyers it is the right call. Twin-wall polycarbonate on an aluminium frame, widely stocked, DIY-friendly, priced roughly $1,500 to $6,000 for the kit. The Glory line uses 10mm twin-wall and carries a 10-year warranty, the Balance and lighter lines run 5 years. It insulates about twice as well as single glass, resists hail, and on a proper base it lasts well past a decade. Where it falls short of a premium glasshouse: the aluminium frame is lighter, the panels sit in channels and can pop out if the greenhouse is under-anchored, the hardware and instructions are fiddly, and the polycarbonate yellows over time. Buy it for value that lasts, not for heirloom looks.

Two hand-built British aluminium glasshouses compared. Warranty, glazing, National Trust range, and why the base is extra on both.

Quick answer: Hartley Botanic and Alitex are the two names at the top of the hand-built aluminium glasshouse market, both British, both bespoke, both built to last a generation. Hartley has made greenhouses in England since 1938, glazes in 4mm toughened glass, and backs the structure with a roughly 30-year guarantee. Alitex is best known for its National Trust collection, carries a 25-year-plus (lifetime on the NT range) structural guarantee plus a 10-year finish warranty, and publishes real starting prices (the Hidcote from about £11,250 including VAT). On both, the masonry base and flooring are extra, pricing is bespoke after a design visit, and lead times are long. Choose on design language, published pricing transparency, and which brand's US service reaches you.

An unanchored greenhouse is a kite. How wind failure happens, what ASCE 7 wind loads mean, and how base fixings and ground anchors keep it up.

Quick answer: Greenhouses fail in wind at the base, not the frame. An unanchored or poorly based greenhouse lifts, racks, and folds, and it is the most common catastrophic greenhouse failure. Anchor the frame to a slab with base fixings, or to a gravel-and-timber perimeter with ground anchors, rated for the local design wind speed that ASCE 7 maps by region and risk category (often 85 to 150-plus mph). Anchoring is one of the cheapest lines in the whole project and the one that decides whether the structure is still standing after the first big storm.

An unvented greenhouse hits 120 to 140F and cooks plants by noon. Vent area, roof and side vents, and auto openers that need no power.

Quick answer: A greenhouse with too little vent area and no automatic openers can cook every plant in one July afternoon, because an unvented greenhouse can climb to 120 to 140F on a sunny day. Fit total vent area equal to roughly 15 to 20 percent of the floor area, split between roof vents and low side vents so hot air rises out while cooler air draws in. Add wax-cylinder automatic vent openers, which open and close on temperature with no electricity and run from about $30 to $60 for a basic unit up to $100-plus for a heavy-duty one (verify at write time). On any greenhouse you will not be standing next to at midday, auto vents are not an accessory, they are what keeps the plants alive.

Wet snow weighs 20 lb per cubic foot. Why a flat cheap roof caves, what ASCE 7 snow load means, and the roof pitch that sheds it.

Quick answer: A cheap flat or low-pitch greenhouse roof caves under wet snow because a shallow roof cannot shed the load, and wet snow weighs up to about 20 pounds per cubic foot. Snow load is rated in pounds per square foot (psf); ASCE 7 sets your ground snow load by location, and a roof at 30 degrees or steeper sheds a large share of it, since the slope factor can cut the effective load by roughly a third on slippery glazing. A greenhouse rated near 25 psf handles roughly 15 inches of wet snow before it is at risk, so in the Snowbelt you want a steep roof, a strong frame, and a rating that clears your local ground snow load with margin. Match the greenhouse to your ASCE 7 ground snow load, not to a catalog photo, and brush heavy wet snow off anyway.

Frost-free runs $10 to $120 a month. A warm growing house runs $100 to $600-plus. What size, glazing, and climate do to your winter bill.

Quick answer: Heating a backyard greenhouse runs from about $10 a month to over $600, and the target temperature drives the bill more than the size does. Keeping a small twin-wall house frost-free at 40F in a mild climate is roughly $10 to $40 a month, while holding a large single-glass house warm enough for winter tomatoes can pass $600. Single-pane glass loses about twice the heat of twin-wall polycarbonate, so glazing and insulation move the bill as much as square footage. For most buyers the honest move is to heat only to frost-free and run a heated propagation bench for seedlings, not to warm the whole structure. Verify current electricity and propane prices for your area, because an energy-cost swing changes every number here.

Tempered glass shatters into blunt granules. Horticultural glass throws sharp shards. Why cheap kits use dangerous glass or brittle poly.

Quick answer: Tempered (toughened) glass is the safe glazing for a greenhouse, because it breaks into small blunt granules instead of the long sharp shards that annealed horticultural glass throws. Horticultural glass is usually 3mm annealed float glass, cheaper and very clear, but a broken overhead pane becomes a falling-glass hazard, which is why premium glasshouses use 3 to 4mm toughened panes. Cheap flat-pack kits cut cost with thin horticultural glass or brittle single-wall polycarbonate that yellows and cracks in two to three seasons. If children or pets are around the garden, choose tempered glass or twin-wall polycarbonate, not annealed.

Gravel-and-timber, concrete slab, or brick dwarf wall? The base runs $500 to $5,000 and decides whether the greenhouse lasts. The honest breakdown.

Quick answer: A greenhouse needs a level, drained, square base, and it is the single most under-budgeted line in the whole project, running $500 to $5,000 or more. Gravel-and-timber ($1 to $3 per square foot in materials) drains well, is DIY-friendly, and suits most mid-market kits. A concrete slab ($3 to $6 per square foot) is the strongest base, best for heavy glasshouses and soft ground, and will not shift or need re-leveling for 25-plus years. A brick or block dwarf wall is the premium base under a glass house, several thousand dollars built by a mason. The base is also what you anchor to, so it decides whether the greenhouse survives a storm.

Too small, cheap glazing, wrong base, no anchoring, no auto vents. The mistakes owners say they would fix first, and how to skip every one.

Quick answer: The regrets owners report most are predictable: buying too small, choosing cheap single-wall glazing that yellows in two or three seasons, setting the greenhouse on a poor base, skipping proper anchoring, and leaving out automatic vents. Any one of these turns a good greenhouse into a frustrating one, and the base and glazing mistakes are the expensive-to-fix kind. Almost none of it is about the brand. It is about the base, the glazing, the anchoring, and the ventilation, the parts nobody puts on the quote. Size up one step, buy twin-wall over single-wall, build a real base, anchor it, and fit auto vents, and you avoid the top five regrets before they happen.

Glass gives 90% light and clarity. Twin-wall polycarbonate insulates twice as well and survives hail. Which fits your climate, honestly.

Quick answer: Glass transmits about 90 percent of light with the clearest view and the longest raw lifespan, but it insulates poorly (around R-0.9 single pane), cracks under hail, and can be a falling-glass hazard unless it is tempered. Twin-wall polycarbonate transmits roughly 80 percent as diffused light, insulates about twice as well (R-1.7), and shrugs off hail and impact, but it yellows over the years and gives no clear view out. For a cold or hail-prone climate and season extension, polycarbonate is usually the smarter call. For light clarity, looks, and a 20-year-plus glasshouse, choose tempered glass. Pick by your climate and how you garden, not by which looks better in a catalog.

Most greenhouses under 120 sq ft skip a building permit, but setbacks, HOA rules, and property lines still apply. The check that stops projects late.

Quick answer: In most US areas a greenhouse under about 120 square feet needs no building permit, but that threshold varies by town and is only half the question. Even a permit-exempt greenhouse still has to obey zoning setbacks, often 5 to 10 feet from a property line, plus easements and any HOA rule on outbuildings. Adding a poured foundation, an electrical circuit, or plumbed water usually pulls it back into permit territory regardless of size. Check your city zoning page and your HOA covenant before you order, because a setback violation or an HOA denial is the line item that halts the project after the kit is already paid for. Confirm your local threshold at write time, since codes differ by municipality.

Twin and triple-wall picks that insulate twice as well as glass and survive hail. Palram, Yoderbilt, and Solexx, honest strengths and weaknesses.

Quick answer: The best polycarbonate greenhouses are the mid-market picks that finally last: Palram/Canopia (twin-wall to 10mm, aluminium frame, $1,500 to $6,000, Glory line warrantied 10 years), Yoderbilt (US-built southern yellow pine with 8mm twin-wall poly, from about $2,695), and Solexx twin-wall polyethylene for cold climates. Polycarbonate insulates roughly twice as well as single glass (R-1.7 twin-wall vs R-0.9), diffuses light evenly, and shrugs off hail that would crack a pane. The tradeoffs are honest: the panels yellow over 10 to 15 years, and a poorly based or under-anchored kit can lose panels in high wind. On a proper base, these grow the same plants as a $20,000 glasshouse for a fraction of the price.

Hartley, Alitex, and Janssens glass against Palram, Yoderbilt, and Solexx. Honest picks by budget, climate, and how you actually garden.

Quick answer: The best greenhouse depends on your lane, not a single winner. For an heirloom glasshouse built to last 20-plus years, Hartley Botanic and Alitex lead the premium tier, both hand-built aluminium with 25 to 30 year structural guarantees and bespoke pricing that starts around $8,000 and climbs well past $40,000 installed. Janssens is the reachable step into real tempered glass at roughly $17,000 to $26,000. For value that lasts, Palram/Canopia twin-wall polycarbonate ($1,500 to $6,000), Yoderbilt's US-built wood-framed houses (from about $2,695), and Solexx twin-wall are the honest mid-market picks. Buy glass for looks and light, buy polycarbonate for insulation, hail resistance, and a smaller bill.

Twin and triple-wall poly, real snow-load ratings, and heating that pencils out. The picks that survive a hard winter and stretch the season.

Quick answer: For a cold climate the right greenhouse is about insulation and snow-load rating first, brand second. Twin-wall or triple-wall polycarbonate (R-1.7 to R-2.8) and Solexx twin-wall polyethylene (R-2.1 to R-2.3) hold heat far better than single-pane glass (R-0.9), which can roughly halve your winter heating bill, and a steep roof sheds snow instead of holding it. In heavy-snow regions the frame has to be rated for your local ground-snow load, so confirm that figure before you buy. The honest short list is Solexx and Palram/Canopia Glory for value and Exaco/Riga or Hoklartherm for a stronger build, each paired with added insulation, a thermostatic heater, and proper anchoring.

Aluminium is low-maintenance and lets in more light. Wood looks warmer and insulates better, but wants upkeep. Which frame fits you.

Quick answer: Aluminium frames are low-maintenance, do not rot, and use slim glazing bars that let in more light, which is why nearly every kit and most premium glasshouses (Hartley, Alitex) use them. Wood frames, cedar at the premium end and pressure-treated softwood like Yoderbilt's southern yellow pine in the mid-market, look warmer, insulate a little better because wood does not conduct heat the way metal does, and feel more substantial, but they want periodic treating and cost more. Choose aluminium for zero upkeep, maximum light, and lower cost. Choose wood for looks, slightly better heat retention, and a more solid feel, if you accept the maintenance. It is a looks-and-upkeep decision, not a quality one.