You can often extend or add to your house without the need to apply for planning permission provided you meet a set of limits and conditions.
The regime for what is allowed without planning permission applies largely to the dimensions of the proposed extension, its position on the house and its proximity to your boundaries.
You will also need to apply for planning permission if more than half the area of land around the “original house” would be covered by additions or other buildings.
NOTE: The extension featured here is for illustrative purposes as typical of many terraced streets. Extensions such as these would not meet the conditions of the permitted development regime and would require planning permission.
Learn more on the Planning Portal.