Where Should You Put a Greenhouse? Check Sunlight Before You Build

The best place for a greenhouse is usually the position with reliable direct sunlight when you will actually grow, enough room to work and ventilate, and no predictable shade from buildings or trees. Do not choose it from a single bright afternoon: a greenhouse that is clear in June can be shaded by a low winter sun, while an open summer site can become difficult to manage without ventilation or shading.

The starting point is a seasonal sun-path check. Use the SunCast Sun Calculator to compare the direction and height of the sun at candidate positions, then inspect the real fences, roofs and trees that could interrupt that path.

Quick answer

Place a greenhouse where it receives the most dependable direct sunlight across your growing months, rather than where the garden looks brightest on one visit. Check winter and summer sun direction, nearby obstructions, access to water and the ability to ventilate before building the base.

Compare sunlight at your candidate sites

Search for the garden, move the map to each candidate area and inspect a few dates and times. The map is a planning prompt: centre it on the exact part of the garden, note the sun’s direction, then look for anything between that direction and the proposed greenhouse.

With two plausible sites, the sun’s direction and elevation for the same date and time tell you which one has a clear line to the sun and which looks straight at the house or a mature tree. The calculator does not know where your roofs, fences and trees are, so that comparison is yours to make on site.

Make winter the first sunlight test

If you intend to grow through winter or start plants early, test the lower-sun months first. A neighbouring roof can block a low winter sun even when a bare tree lets most of it through.

Latitude decides how hard this test is. On 21 December the sun reaches 16° at noon in Exeter and only 11° in Edinburgh. It stays above 10° for about four and a half hours in Exeter and about an hour and a half in Edinburgh, and rises and sets well round to the south-east and south-west in both. A 1.8 m fence due south casts a noon shadow 6.3 m long in Exeter and 9.6 m in Edinburgh. By 15 February, when many gardeners start sowing under glass, the noon sun is 27° in Exeter and 21° in Edinburgh, and the fence shadow shrinks to 3.6 m and 4.6 m. (All figures calculated with SunCast’s sun calculator, on level ground.)

So a Scottish greenhouse for early sowing needs a clear view low to the south, from roughly south-east round to south-west. A neighbour’s two-storey house 15 m away in that direction can be enough to shade it in midwinter.

At each candidate position, ask:

• Where is the sun during the first useful part of the day?
• Which roofline, fence, tree or hedge sits in that direction?
• Does the obstruction block the sun at low elevation, or only when it is high?
• Does the shadow move across the whole greenhouse footprint, or only a corner?

The guide to measuring full sun, partial sun and shade explains why open-sky brightness and direct sunlight are different. A greenhouse can still feel light in indirect daylight while receiving too little direct sun for the crops and season you have in mind.

Then check summer heat and shade

More sunlight is not the whole brief. In hot weather, a glasshouse can require ventilation and temporary shading. The RHS greenhouse ventilation and shading guidance recommends both to help control summer temperatures.

Plan for that heat when you choose the site, without giving up winter light for it. The site needs accessible doors and vents, a sensible path, a water source and a way to add temporary shade when conditions call for it. A tree that shades the greenhouse may also shed leaves, drip water or grow; do not assume its current canopy will remain the same.

A five-step siting checklist

1. List the growing months and crops

Be specific. A greenhouse used for tomatoes from late spring has a different light and heat problem from one used to start seedlings in late winter. Write down the months first, then test those dates in the calculator and outdoors.

2. Mark two or three possible footprints

Use string, canes or a hose to outline the full greenhouse and the working space around it. Include the door swing, water route and space needed to clean glazing or reach vents. A sunny gap that cannot be used comfortably is not automatically the best site.

3. Observe direct sunlight

On clear days, note when each footprint enters and leaves direct sun. Photograph the shadow edge at the same positions. The garden sun-exposure guide provides a simple observation method; return on another date instead of treating one observation as a year-round result.

4. Compare the sun path by season

Check a date near the beginning, middle and end of the growing period. Solar azimuth tells you the direction from which light arrives; elevation tells you how high it is. Earth’s tilted axis changes both through the year, as NASA explains. June and December are useful contrasts, but the seasons are opposite in the two hemispheres.

5. Decide how heat and access will work

Before laying a base, walk the watering route and imagine opening the door on a hot day. Check drainage after rain, available electricity if you need it, and how tall crops or nearby supports could cast their own shade. Keep the greenhouse clear of branches that could damage glazing.

Compare sites using the same criteria

Use a small table to make trade-offs visible. Fill in each cell from your own observations.

Criterion	Beside the patio	Back of the garden
Direct sun in the intended months	Record dates and hours	Record dates and hours
Winter roof or hedge shade	Check low-sun direction	Check low-sun direction
Summer ventilation and shading	Note doors, vents and space	Note doors, vents and space
Water and working access	Measure real route	Measure real route
Future obstructions	Trees, extensions, supports	Trees, extensions, supports

This stops a single attractive feature, such as being near the kitchen, from hiding a serious seasonal-light issue. If the two sites are close, choose the one with the clearer evidence and the easier day-to-day routine.

Common siting mistakes

• Using total daylight instead of direct sun. Long summer days do not prove that sunlight reaches the greenhouse.
• Ignoring the mature garden. A young tree, hedge or climbing support may be a much larger blocker in a few years.
• Testing only at noon. Morning and afternoon shadows can matter more than the short midday peak.
• Building before testing access. A greenhouse needs watering, cleaning, ventilation and room to work; a difficult route becomes a daily nuisance.
• Treating a map as a complete shadow survey. Map and solar data help you screen choices, but site observation catches the smaller objects and local conditions that a model misses.

Start by comparing your two most plausible positions in the Sun Calculator, then spend a clear day observing the winner.

Frequently asked questions

Where is the sunniest place to put a greenhouse?

Choose a position with the most reliable direct sunlight during the months you plan to grow, while leaving practical access and room for ventilation. The apparently sunniest patch today may be shaded by a building, hedge or tree at another time of year.

Should I check greenhouse sunlight in winter or summer?

Check both. Winter light is often the limiting condition for year-round growing, while summer checks reveal heat and shade patterns. Use the exact months in which you expect to use the greenhouse rather than treating either solstice as a complete answer.

Can a sun-position calculator tell me whether a greenhouse will be shaded?

It shows where the sun will be by date and time. Compare that direction with real buildings, trees and fences at the site; a basic sun calculation does not automatically map every obstruction or the greenhouse itself.