Crown Reduction is normally required when a tree has grown too large for its permitted space. A long term solution, and often the best solution, is to remove the tree completely and replant. If crown reduction is necessary, it is often required when a tree or part of a tree is touching or very close to a building.
Crown reduction should be considered in order to achieve an overall reduction in the volume of a tree. Work usually involves a general shortening of minor branches. Generally, 20% is the maximum that can reasonably be taken from a crown.