Measure from the original house
For older houses, the 'original' house is generally the house as first built or as it stood on the relevant historic date used by the rules. Later additions do not reset the starting point.
Boundaries and eaves matter
Height near boundaries, projection beyond original walls and total land coverage are common limits. Corner plots can create additional questions because a side wall may front a highway.
Do not rely on a neighbour's project
Two apparently identical houses can have different planning histories or restrictions. One neighbour's extension is useful context, not proof that yours is permitted.
Get the position confirmed before building
Where permitted-development status matters to a future sale or lender, consider obtaining formal confirmation such as a lawful development certificate where that route exists.