Heritage Open days 2010 takes place 9th-12th September.
Heritage Open Days celebrates England’s fantastic architecture and culture by offering free access to properties that are usually closed to the public or normally charge for admission.
Open House London, "the capital's greatest architectural showcase", will take place on 18 & 19 September 2010. You can order the events booklet online or, if you live in London pick one up from your local council - they were in the civic reception at work.
There are usually somewhere locally to go, I will be checking out the London booklet later!