There are several reasons why UK planning applications may be rejected, including:
1. The proposed development does not conform to local planning policies or guidelines.
2. The proposed development would have a negative impact on the surrounding area or on protected species or habitats.
3. The proposed development is not in line with the local authority's development plan.
4. The proposed development is considered to be overdevelopment of the site.
5. Insufficient information or technical details were provided with the application.
6. The proposed development would cause unacceptable levels of traffic, noise, or pollution.
7. The proposed development would have a negative impact on the character or appearance of the area.
8. The proposed development would not provide adequate parking or other necessary facilities.