The NPPF states that “the replacement of a building, provided the new building is in the same use and not materially larger than the one it replaces”.
While there is no definitive legal definition of “materially larger” there have been many appeals and legal cases which try to ascertain what the term means and what scale should be allowable.
In addition, each Local Planning Authority will have their own definition of what they would consider “materially larger” when considering green belt replacement homes, but often these range from around 15 – 30% from the existing dwelling.