Nc for this out of paranoia 
Not sure whether to post here or legal but hopefully someone can offer some guidance...
We have just (in the last year or so) completed a second floor extension on our house which is a leasehold property. Prior to this, my husband (before I was on the scene) also extended the small kitchen into a much larger one.
For both extensions, we have failed to obtain permission from the leaseholder (which is a large property company).
To be honest I was completely unaware of this issue and my husband had buried his head in the sand over it.
Of course we are now panicking and wondering how to put it right so that it isn't an issue if we decide to sell later down the line. We've enquired about purchasing the freehold but apparently that doesn't remove the restrictions around the extension. Not sure whether to approach them to get backdated approval as we want to know what they could throw at us for failing to do this upfront.
We've read online that they could charge us 50% of the increase in value of the house?!
Does anyone have any experience of this?