house extentions

[Malagabob333]

Is there a minimum percentage of area you can extend your property without needing to apply for planning permision

[Ethel]

10% of the original volume if terraced or in a conservation area, 15% if detached or semi-detached MAXIMUM

[buildersmate]

..... unless you've already extended the property, because the limits apply to the house AS ORIGINALLY built or as at 1948 (if the house is older).
This is called using your Permitted Development Rights.
Some dwellings have has their permitted development rights withdrawn - particuarly dwelling in a national Park and some Conservation Areas. Ask the planners at your council if in doubt. They won't fib to you.