We've decided to put an offer in on a house we viewed last week. A massive benefit is that it has a back garden which backs directly onto a massive park. All green space, couple of football pitches, kids park etc.
To get to the park from the front of the house is only a two minute walk...but I would love to put a gate in the back fence to go directly from our back garden into the park. I have a vision of the dc just being able to wander directly into the park, maybe me sitting in a chair on the boundary and throwing a tennis ball for ddog into the park without having to leave the house! 
There are no other examples where this has been done - the other 15 or so houses on the same side of our road have a lane running across the back of their gardens, which peters out a few feet before our house, so we're the only garden with possible direct access.
The fence is a mish mash of different types/heights along the border of the park and the fence between our garden and the park is different to the rest - but because it's a mish mash anyway, it's impossible to tell whether the fence is ours or belongs to the council.
I have a suspicion I should probably ask the council, but i'm afraid they'll give a blanket no...so i'm tempted to just do it, and hope no one notices or cares. BIL is a carpenter so could do it for us, using the fence panel wood - all very inconspicuous and natural.
Anyone know anything about this? What's the worst that could happen if we were found out?