The presence of breeding birds on a development site will affect the timing of works. Since the nests are also protected, any work that may affect birds should be undertaken outside of the nesting season when birds have a greater ability to move elsewhere. The nesting season is taken to be between March 1st and August 30th, but warm weather in late winter may encourage birds to nest as early as February.
Birds will nest in tree branches and tree holes, hedges, inside buildings and barns, under soffit boards or on the ground, depending on the species. A development needs to avoid direct or indirect disturbance to these places.
All developers should read Planning Advice note 21: Barn Owls before submitting an application that may affect barn owls.
A number of different nest boxes, photos of barn conversions including barn owl boxes and other useful information is available from the Barn Owl Trust website which also includes advice for barn conversions.
English Nature (now Natural England) has produced a guide for Developers and Planners for Barn Owls on Site. While much of this information will still apply in Wales, please note that licensing arrangements are different and also consult the Countryside Council for Wales rather than Natural England.
A guide to Birds and the law is available from the RSPB.