Planning permission % sizes

[skateboarder]

We are just at the 'thinking about an extension' phase and wonder if any lovely mn's can help.
Previous owners extended house 5 years ago. We think to about 45% of original size. (was 2 up, 2 down type property. Now 4 up, 3 down)
We have heard there is a maximum % that planning permission can be granted for. What is this? Is there any movement in this at all?
We would be looking to add a single storey extension (poss 6m x 4m), I'm guessing this would take us to about 60% of original property.
Any advice or should we can it and think about moving? which I don't really want to do.

And my title makes no sense whatsoever

Are you detached or attached?

Are you in a conservation area or a listed building?

Would the proposed extensions take up more than 50% of your outside space.

Did they do the extension as permitted development or obtain planning?

We are semi detached but not in the usual way; the back of our property is attached to a non converted barn.
We are not in a conservation area, green belt though.
Not a listed building.
The extension would not take up more than 50% of the a) original outside space or b) remaining outdoor space.

Are all the points you raise indicators of whether pp may or may not be granted?

Obtained planning in 2005 I think

There are outline rules about allowable rear extensions. For a terrace or semi or link detached in England it's 3m depth no clear width limit. Considerations need to be made for windows as there are rules about overlooking etc. this is what is allowable under permitted development (without planning)

Most planning departments offer monthly clinics where you can take in a fag packet sketch and they will direct you to how that falls within the local plan. They won't give you a yes but if its a fairly likely no, they'll tell you. Also they'll help with what you could change to make it more favourable.

thank you misformum your help is useful.
Unfortunately we do not have any space at the back of the property, our space is to the front and sides.
I will drop the planning department a line and see if my fag pack sketch would be considered. I strongly suspect not; but I will ask the questions and see if there is any deviation allowed.

Thanks again.