Planning consultancy Green Balance was commissioned by the trust to look at 15 planning decisions from the past three years. It found in many cases planning rules were not correctly applied, often due to pressure to provide housing.
The National Trust said: "The planning system is supposed to steer development towards where it's most appropriate and can provide most value while protecting other areas for their landscape or wildlife. But with local planning authorities losing staff and expertise, and being pressured to make decisions in favour of development, that practice is sometimes falling below what it should be."
The trust has suggested ways that practice could be improved, including a series of nine questions that should be asked by those making planning decisions affecting AONBs.