A few places spring to mind.

Cragside []

Although there are large parts of the grounds and house too steep for wheelchair access, this warrants a mention largely because of the attitude of the staff towards those with less than full mobility.
There are ramps and disabled toilets present but someone who is able to walk short distances will get far more from the experience. The famous lift is not only present, but still in use. The mechanism has changed, and it’s not what you’d call spacious, but if you’re in a wheelchair, staff can take you between floors. [It’s worth mentioning, this treat is not available to the fully abled]

Hardwick Park []

This is a great day out and wheelchair accessible, at least everywhere we went, with disabled toilet facilities. It’s tranquil with good tourist information and the gift shop has a good selection. It’s worth noting that the park covers quite a distance so it’s best using the facilities to avoid getting caught short. [Every time we’ve been there’s been a young child ‘watering’ one of the many bushes]. Some of the paths camber for a good distance.

Bolam Lake []

Bolam Lake boasts accessibility. Even my stubbornness managed only a couple of hundred yards before being forced to leave. The tree roots make the paths difficult to navigate. If you’re in a wheelchair because of excessive pain, I’d recommend somewhere else.

Hope these help.