How to Build a Cold Frame: Step-by-Step UK Guide

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Key takeaways

A cold frame is a simple glazed box with a sloping lid that traps the sun’s warmth, protecting seedlings and extending the growing season at both ends of the year.
Building your own from pressure-treated timber and twin-wall polycarbonate typically costs £40–£80, compared with £90–£200 for many shop-bought versions, according to UK Patio Garden.
A basic 1m × 60cm cold frame is a beginner-friendly project taking roughly half a day (three to four hours).
No planning permission or concrete is needed; it simply sits on level ground, ideally angled toward the south.
Position matters and so does ventilation: on sunny days you must prop the lid open or you risk cooking your plants.

If you want to protect seedlings and add weeks to your growing season without spending much, build a cold frame: a simple box with a transparent, sloping lid that traps the sun’s warmth and shelters plants from wind, rain and frost. This guide walks you through how to build a cold frame from treated timber and polycarbonate in a single afternoon, then explains how to use it well through the British year.

What exactly is a cold frame and what does it do?

A cold frame is, in the RHS’s words, a box that lies flat on the ground with a glazed, sloping lid. There is no heater and no electricity; it works entirely by trapping the sun’s warmth and sheltering whatever is inside, which makes it an unheated mini-greenhouse (UK Homesteading).

Despite its simplicity, it earns its place several times over. A cold frame lets you:

Harden off seedlings, gradually acclimatising plants raised indoors before they go into the ground.
Overwinter tender plants and bring on autumn sowings of hardy salad leaves and herbs.
Extend the season, sowing spinach, lettuce and winter salads weeks earlier and later than you otherwise could (UK Patio Garden).

According to The Garden Patch, a good cold frame can extend the growing season by four to six weeks at both ends of the year. If you raise plants from seed, it is the piece of kit that stops all that windowsill effort being undone the moment your seedlings meet the open garden.

How to build a cold frame: what you’ll need

Costs vary depending on what you use. UK Patio Garden puts a DIY build at £40–£80, compared with £90–£200 for many shop-bought models, while The Garden Patch suggests £20–£60 in materials is achievable. If you can get hold of scrap timber and an old window, UK Homesteading notes the classic version costs next to nothing.

For a basic 1m × 60cm cold frame you will need:

Pressure-treated timber for the sides and corner posts. Treated timber resists rot and will last years outdoors.
Twin-wall polycarbonate sheet for the lid. It is lighter and safer than glass, and insulates better than single sheets.
Outdoor wood screws and a drill/driver.
Hinges (two is plenty) so the lid can be propped and held open for ventilation.
Wood preservative or paint for cut edges and the interior.
Basic tools: saw, tape measure, set square, spirit level and pencil.

Wood is by far the most common material, according to Horticulture.co.uk, although brick and plastic are both viable options. This guide focuses on wood because it suits beginners and standard tools.

Step 1: Choose the right position

Before you cut anything, decide where the frame will live. The design has barely changed in two hundred years because the standard shape works: a sloped timber frame with a glazed lid tilted toward the south, lower at the front and higher at the back. The slope sheds rainwater and angles the lid toward the winter sun to maximise light capture (The Garden Patch).

Pick a south-facing spot that gets good winter light.
Choose level ground; the frame sits directly on the soil, a patio or a paved base with no foundations.
Avoid deep shade from fences, walls or evergreen shrubs.
Think about access, because you will be opening the lid daily in sunny weather.

If your chosen spot is on soft ground, a firm, level base makes the frame last longer. Our guide to building a shed base covers similar principles for levelling ground in a UK garden (see our shed base guide).

Step 2: Cut the timber

The back of the frame must be taller than the front to create the slope. As a worked example for a 1m × 60cm frame, try this:

Back panels: two pieces of timber 30cm tall, cut to 1m long (one for each end, or enough for a back rail plus corner posts).
Front panel: timber 15–20cm tall, cut to 1m long.
Side panels: cut with a sloping top edge from 30cm at the back down to 15–20cm at the front, 60cm deep. Set your saw at the matching angle, or mark the slope with a straightedge and cut freehand.

The exact dimensions matter less than the principle: the slope should be noticeable, roughly one part drop in six to eight, so rain runs off and winter light enters at a useful angle. Check every piece with a set square as you go; small errors compound quickly at the assembly stage.

Step 3: Assemble the box

Fix the corner posts first if you are using them, screwing the back and front rails to them at the right heights.
Attach the side panels, aligning their sloping top edges with the tops of the front and back. Pre-drill to avoid splitting the timber near the ends.
Check for square by measuring corner to corner in both directions; adjust before the screws are fully tightened.
Seal and treat all cut edges with preservative, especially on untreated timber.

Take your time at this stage. A rigid, square box makes fitting the lid straightforward; a twisted one makes it miserable. If you have built flat-roofed structures before, the joinery here is very similar, and our shed roof guide covers the same sloped-lid logic at a larger scale (how to build a shed roof).

Step 4: Build and fit the lid

Cut the polycarbonate slightly larger than the top opening so it overhangs the frame and sheds water away from the timber. Screw through battens laid across the top of the box into the lid frame, or make a simple lid frame from timber and fix the polycarbonate to that before hinging it along the top back edge.

Fit the hinges so the lid can open fully and be held at various angles; a simple prop stick works well.
Position the twin-wall polycarbonate with its flutes running down the slope so condensation and rainwater can drain.
Seal the top edges of the sheet if the manufacturer recommends it, to keep dirt and moisture out of the flutes.

A simple hinged roof is what allows airflow and temperature to be controlled to the benefit of plant growth, as Master Horticulturist Colin Skelly explains on Horticulture.co.uk. In other words, the lid is not just a window; it is your main temperature-control tool.

How do you use a cold frame through the British year?

Building the frame is the easy half; using it well is where the value is. UK Homesteading stresses that the goal is warmth without cooking your plants.

Spring: hardening off and early sowings

Use the frame to harden off seedlings raised indoors or in a greenhouse, opening the lid a little more each day over a week or two until plants can cope with the open garden. It is also ideal for early sowings of lettuce, spinach and other crops weeks before they could survive outside.

Summer: mostly a ventilation job

On sunny days the temperature inside can climb sharply, so prop the lid open, and on hot days open it wide or remove it entirely for tender plants. Forgetting to ventilate is one of the most common ways gardeners lose plants in a cold frame, so check the lid regularly, especially on sunny days.

Autumn and winter: protection season

Overwinter tender plants and bring on autumn-sown hardy salads and herbs, as recommended in UK growing guides. On very cold nights, an old blanket or bubble wrap laid over the lid adds insulation; remember to remove it in the morning.

Common cold frame mistakes to avoid

Never opening the lid. Trapped sun can cook seedlings even on cool days.
Putting it in shade. The frame needs low winter sun, so a south-facing spot is best.
Using untreated timber. Guides such as UK Patio Garden recommend pressure-treated timber for a frame that will stand up to damp conditions, and any cut edges should be treated before use.
Overcrowding. Air needs to move around plants, or damp and disease follow.
Skipping the slope. A flat lid pools water and captures less winter light.

Want more self-sufficiency projects like this?

A cold frame is a satisfying afternoon build, but it is often just the first step for gardeners who want to grow more of their own food. The Self Sufficient Backyard guide is a digital product marketed by its creators as a library of projects and techniques for a more self-reliant garden and homestead, according to the seller. Independent verification of those claims is limited, so treat it as a source of project ideas rather than a promise of outcomes. It suits readers who enjoy DIY and want a structured set of plans to work through; it is less relevant if you only want the occasional small project like this one. Free alternatives exist too: libraries, gardening charities such as garden organic groups, and the many free UK guides published by sites like the ones cited in this article.

FAQ

How much does it cost to build a cold frame in the UK?

Expect roughly £40–£80 for a DIY build in treated timber and polycarbonate, according to UK Patio Garden, though The Garden Patch suggests £20–£60 is achievable. Using scrap timber and an old window can bring the cost close to zero. Shop-bought frames typically run £90–£200.

Do I need planning permission for a cold frame?

No. A cold frame is a small, ground-level structure with no foundations or fixed services, and UK Patio Garden confirms it needs no planning permission or concrete. Just keep it on your own property and out of the way of paths and doors.

Can I use an old window as the lid?

Yes, and it is the classic budget approach noted by UK Homesteading. Check the glass is sound and consider whether it is safe around children and pets; twin-wall polycarbonate is lighter and shatter-resistant if safety is a concern.

When should I open the lid of my cold frame?

Open it on any sunny day, even cold ones, because internal temperatures can climb surprisingly high. Adjust how far you prop the lid with the weather and the season, opening it wider in milder spring conditions and keeping it mostly closed in winter while still allowing some airflow on sunny days.

What can a cold frame realistically do for tender plants in winter?

It offers frost protection, wind shelter and a few degrees of extra warmth, which is usually enough for hardy salads, herbs and overwintering tender or half-hardy plants. It is not a heated greenhouse, so truly tropical plants will still need indoor space through the hardest weather.

Final thoughts

A cold frame is one of the most cost-effective structures a UK gardener can build: half a day of straightforward joinery buys you a sturdier result than many shop-bought models, at a lower price, sized exactly to your space. Start small, keep the lid propped on sunny days, and you will wonder how you managed without it. Once the frame is up, the natural next step is sowing your first early salads and, if you are bitten by the DIY bug, planning a slightly bigger garden build for spring.

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