Is it true that if an addition/extension to a house is less than 10 % of the property, you don't need planning permission?

[emkana]

Am thinking I read this somewhere?

Yes it's true - if you don't increase the volume of your house by 10%. But also wise to check with your local planning department!

Generally yes - but it's by volume not area

Plus if any previous addition have been added (porch, conservatory, garage, large shed) that takes up the allowance

I think you're right, but remember that a garage or conservatoty may have been built later than the house - our garage counted having been built in 1950, 20 years after the house!

If you have a 3 bedroom semi or bigger it is up to 70 cubic metres of visible external building. Or for smaller dwellings, 50 cubic metres.

But, there may be exceptions so check with your planning authority. They probably have the details on their website.

Oh, and the allowance is called permisible development.