You will need a planning permission if you are planning to:
- build something new
- make a major change to your building
- change the use of the building
But there are some jobs, which would not require a planning permission even if they go under the point - building something new. These kind of jobs are called Permitted Developments (PD).

Permitted Developments include:

1. Internal Remodelling
If you are considering to change the internal layout of the property by removing or changing wall position, you would be able to carry out the work without obtaining full planning permission. In this case, you will need to follow Building Regulation guidance for such aspects as structural alterations and electric works.

2. Moving Windows & Doors
4. Single Storey Extensions & Conservatories

You would not need a planning permission to replace windows in the original walls of the house. For new or bigger windows or doors, you will need to follow Building Regulation guidance.

3. Using Attached Buildings, e.g. Garages
You can do internal remodelling of existing space under PD, providing you are not increasing the overall footprint of the dwelling.

You would be able to build an extension without Planning Permission if:
- it does not sit forward of the principal elevation
- new building materials are matching existing
- rear extensions are no more than 4m in depth for detached houses and no more than 3m for semi-detached or terrace
- side extensions' width should not be more than half width of the original dwelling. Side extensions are not permitted on Article 1(5) Land (e.g. AONB, Conservation Areas). 

5. Loft Conversions
Loft conversions provide a good solution for enlarging square meters of the property and may not require a planning permission. Permitted Development allows for the construction of dormer windows in the converted attic. Dormer windows should not sit higher than the highest point of the existing roof.

6. Two Storey Extensions
A two storey extension can only be built under PD if it is at the rear of the house and no longer than 3m. Some restrictions may apply to the glazed nature of windows in these extensions. 

7. Sheds & Outbuildings
Outbuildings should not exceed 50% of the total area of the curtilage and they cannot be built in front of the house. Height restrictions depending on the type of roof (4m for dual pitch roofs, 3m for other roofs, and 2.5m when the building is within 2m of the boundary). Outbuildings may only be single storey, with the maximum eaves height remaining at 2.5m. Under PD outbuildings cannot be used for residential accommodation, e.g bedrooms.
8. Converting Two Homes Into One
Converting two houses such as pair or semis into one could be done under PD. However, if you would like to divide one house into two flats you will then need to apply for planning permission.

9. Porches
The porch or any if its parts should not be taller than 3m and the ground area should not exceed 3m².
10. Decking
Decking would not require a planning permission if the raised platform height is not greater than 300mm. 

11. Swimming Pools
You can build a swimming pool under PD if the total area covered by the pool does not exceed 50% of the area 

For more information regarding specific type of building works check planning portal.
Source: Homebuilding & Renovating