Nest boxes: hole sizes, height and when to put one up
The BTO’s hole sizes, including where two BTO pages disagree, plus height, shelter and the February prospecting window.
A nest box replaces a hole that a tidy garden no longer has. The BTO’s nest box pages are the source for sizes and siting. Two of those pages, both open on 14 November 2026, do not say the same thing about the hole. This guide reports both.
Hole size: two BTO pages
On Make a nest box the BTO recommends:
| Hole | Species they list |
|---|---|
| 25 mm | Blue tit, coal tit, marsh tit |
| 28 mm | Great tit, tree sparrow |
| 32 mm | House sparrow, nuthatch |
| Open front | Robin, spotted flycatcher |
They note that a smaller bird can use a larger hole, and that a larger species may then take the site. Starlings, on the putting-up page, are given a 45 mm hole. House sparrows are also described there with a 32 mm hole on a house wall.
On Buying a nest box the same organisation says choose 32 mm as the size for small hole-nesting birds such as sparrows and tits, and choose 28 mm only if you want to restrict the box to blue tits. They also say holes smaller than that will be too small, which sits awkwardly next to the 25 mm row on the make page.
We are not papering over that. If you are drilling one hole, read both pages and decide which birds you are willing to let in. A 32 mm hole, on the buying page’s logic, keeps the box open to tits and sparrows. A 25 mm hole, on the make page’s table, is the blue tit, coal tit and marsh tit size, and is more likely to exclude great tits.
The rest of the box
From the buying page:
- Wood, or a wood-concrete compound. Not a dense material such as metal or ceramic, which they say can make the inside too hot or too cold.
- Wooden walls at least 15 mm thick. Untreated is not required: they say the outside can have a non-toxic, water-based preservative. Cedar, oak or beech outlast pine.
- Internal floor at least 130 square centimetres. They say birds may lay fewer eggs in a smaller box.
- No perch. It gives a squirrel or a weasel a place to stand.
- A front or a roof you can open, so the box can be checked and cleaned. Do not buy one built into a bird table.
The make page says hinge the roof, with a waterproof strip, so you can lift it without it falling off.
Where and when
From the putting-up page:
- Not too close to another box of the same type. Neighbours fight.
- Front vertical or tipped slightly down, so rain does not run in. Shelter from the prevailing wind, rain and strong sun. The page does not say “face it north-east”. That sentence is not here because it was not on the page.
- Small-hole boxes about 1–3 m up on a trunk, entrance clear of leaves, or on a shed or wall if there is no tree. Out of easy reach of cats and squirrels.
- Open-fronted boxes for robins on a wall or fence that already has shrubs or creepers, hidden rather than in the open.
- Away from feeders. A busy feeder disturbs a nesting pair.
- Fix with galvanised or stainless screws, or galvanised wire on a tree, and check the fixing as the tree grows.
Pairs of many small birds start looking in the latter half of February, so a box up at the end of winter is in time. The BTO also says it is never too early or too late, because birds roost in boxes in winter. For robins in an open-fronted box, nest building can start in February or March.
If nothing uses it, they say the reasons include natural holes nearby and territory boundaries. A vacant box for a year or two is common. Moving it is reasonable if several seasons pass with no interest.
A metal plate around the hole is their suggestion against squirrels enlarging the entrance.
Cleaning, and the law on active nests, is a separate guide: when you may clean a nest box.
Which hole should I drill if I only want blue tits?
The buying page says 28 mm if you want to limit the box to blue tits. The make page lists 25 mm for blue, coal and marsh tits, and 28 mm for great tits. Those are both BTO pages. Read them before you drill.
How high?
The BTO says 1–3 m for a small-hole box, and “about 3 metres” as the heading above that sentence. Clear flight path, not buried in leaves.
Will a camera box with a 38 mm hole suit blue tits?
A 38 mm entrance is larger than every small-bird hole on these BTO pages. The camera guide includes a maker’s box stated as 38 mm with no extra guards. That is the maker’s figure, set next to this table so you can compare them.