Building a Greenhouse

What do I want the greenhouse to do?

Decide whether you want to start seedlings, extend the season for salad crops, grow warm-season plants earlier or overwinter tender ones. A seed-starting house can be small and lightly heated, while one that grows crops through winter takes more heat, light and attention.

Be honest about time. A greenhouse needs daily checks in spring, when temperatures swing, and a neglected one can cook plants in a few hours of sun.

A greenhouse is also a place where mistakes happen quickly. Sun can scorch plants, humidity can invite disease and a power outage can end a heated season overnight. Plan a simple backup, such as a way to vent passively or move tender plants, before you rely on the space.

Do I need a permit?

Rules vary by state, county and town. Ask your local building or zoning office whether a greenhouse of your size needs a permit, how far it must sit from property lines, and whether there are height or electrical requirements. Agricultural exemptions, where they exist, depend on local definitions, so do not assume one applies.

If you plan to add electricity, gas heat or running water, ask what inspections are needed. Electrical and gas work should be done by licensed professionals where rules require it.

Ask neighbors who run greenhouses what surprised them in their first year. Their answers about wind, heat and pests are specific to your area and often more useful than any general advice.

Where should I site it?

Choose a spot with plenty of sun through the cold part of the year, with level, well-drained ground and shelter from strong wind. Avoid shade from buildings and trees, remembering that the sun sits lower in winter. Keep it within reach of water and power if you will need them.

Think about snow slide from the house roof, runoff from slopes and access with a cart. A site that is easy to reach is a site you will actually use in bad weather.

What size and frame should I choose?

Many first greenhouses end up feeling too small. Allow room for benches, a walkway and storage, and consider building a size that can be extended. Frames come in wood, metal and plastic pipe, and each has different cost, durability and wind and snow resistance.

Check the loads your climate can put on the structure, especially wind and snow, and follow a tested plan or the kit maker's instructions rather than improvising. A solid foundation or anchoring prevents the structure from lifting or shifting.

Think about the floor as well. Gravel, pavers or packed soil all work, but standing water inside leads to humidity problems and slippery footing. Put in a path wide enough for a cart, and design benches at a height you can use without stooping.

• Room for a walkway and benches
• Frame suited to local wind and snow loads
• Firm anchoring or foundation
• Doors wide enough for a cart

Which covering is best?

Glass, rigid panels and plastic film each have trade-offs in cost, light, heat retention and lifespan. Film is cheaper but needs replacing, and panels offer more insulation. Think about hail, snow and the availability of repair parts.

Ask your extension office which coverings have worked locally and whether a double layer is worth the cost in your climate.

How do I handle ventilation, heat and water?

Ventilation is the most important feature. Roof vents and side or end vents let hot air escape, and automatic openers that need no electricity can protect plants when you are away. Add shade cloth if summer sun is strong. Good airflow also reduces mildew and other humidity-related disease.

If you add heat, follow the heater's instructions and local codes, provide fresh air as required and never use unvented fuel-burning heaters in an enclosed space, since they can release dangerous gases. Install a thermometer that records highs and lows, and keep electrical devices rated for damp conditions.

How do I keep plants healthy inside?

Clean benches and pots between uses, and inspect new plants for pests before bringing them in. Water in the morning so leaves dry before night. Keep records of temperature and what you planted so you can learn from each season.

For pest and disease questions, ask your extension office before using any product, and follow the label exactly.

Keep a record of what you grow, when you sow and what temperature swings you see. Your extension office can help you choose crops that suit your light and heat, and what pests to watch for in an enclosed space.

Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.