Barn Owl Box – Essential Size Guide

If you build your own Barn Owl Nest box, step one is to get a list of the ideal sizes you need for the barn owl nest box to succeed. The box you make needs to appeal to the owl and be big enough and safe enough “by design” for the owl’s welfare.

I’ve created a guide and checklist for building my barn owl nest boxes. I hope you find it useful.

 Barn Owl Box – Sizing Check List

• Entrance hole size : Optimum size 130 x 130mm; minimum 100 x 100mm; maximum 150 x 150mm

I’ve used a 140mm hole saw, and it’s perfect for the owls to get in—not too small and not too big. Tawnyys can still squeeze through this size, but they prefer a much bigger hole, so it does help deter them. 

• Entrance hole height: The distance from the bottom of the entrance hole to the nest floor must be ideally 450mm or more. This helps prevent the owlets from falling out of the box when they start jumping and practising flying. 

I’ve included an internal viewing perch on one of my taller barrel designs so adults can see out. They do enjoy that, and it encourages them to use the box. On another shorter design, I’ve put a small perch on the box floor so adults can see out a little if they want to. 

• Nest Floor area* This is the bottom internal floor where the barn owls will lay the eggs and raise the owlets. Sometimes called “the scrape”: Good size ranges from 0.25 to 0.5 m2, with an absolute minimum of 0.2 m2, but the aim is to reach around 0.4 m2.

The oak Barrels I’ve used to create nest boxes have a floor area of approximately 0.4m2. To get this size, you will need a barrel with a diameter of around 700mm, which will be the floor area.

• An external landing platform.  Below the entrance hole, it’s vital to have a platform that the adult birds can land on when returning to the nest with food, and also it’s a platform where the owlets will first see the outside world when they venture out of the nest for a look around.  Think of this external landing platform as “a balcony for the owlets”. So that it’s safe, it needs to be of a minimum size and also have a small lip so the owlets don’t fall off it.

The external platform is vital for the safety of young fledglings. Climbing or jumping, young birds can get from the platform onto the roof of the box and (ideally) onto other nearby perching places. It helps and encourages adult barn owls to visit the box and ultimately nest in it, too. Minimum floor area 0.125m2 (20cm x 50cm), minimum lip height 10cm

Fit for Purpose Barn Owl Box checklist.

• Human access for easy clearing out of nest debris is essential. I’ve used hinged lids to do this.

• The interior must remain dry during prolonged heavy rain coming from any direction. So, make sure the wood or building material is waterproof.

• All sides should overhang the floor to make the box as watertight as possible. Outdoor nest boxes will need to have drainage holes just in case water gets in, but the box must be designed so that water does not enter it in the first place.

• The roof should be covered in thick roofing felt or waterproof material that will last at least 5-10 years. I’ve used solar panels on some boxes, which add waterproofing.

• Timber liable to decay should be treated with a long-lasting natural preservative (bees wax is good), pressure-treated (tanalised), or surface-treated. I’ve used natural oak, which requires no preservative. 

• All screws/nails and any metal fittings used should be rust-proof.

• Barn Owl boxes in trees should be installed at 10-16ft high. 14ft works well to attract barn owls. Too low and it can get more predators like stoats.

• Position your box so that a barn owl can see and easily fly through the opening without obstructions such as leaves and branches.

• The box’s entrance hole should be positioned away from prevailing winds and driving rain. Depending on the location, the prevailing weather can be different. Personally, my boxes with an entrance hole pointing northwest have been the most successful in attracting barn owls and keeping the boxes warm and dry from prevailing weather.

• Your barn owl box needs an entrance hole that ideally overlooks rough grass that the owl will use to hunt on the wing for mice and voles. This land area needs to be at least around 1 acre or more to attract a barn owl. Ideally 2acres+