Complex sentence: The five-bedroom house had been bought by Karen and Richard for their parents in 1970, and it was in an annex to the house (subsequently pulled down) that Karen collapsed from the heart attack that killed her in 1983.
Split into simpler sentences: The five-bedroom house had been bought by Karen and Richard for their parents in 1970. It was an annex to the now-destroyed house in which Karen collapsed from the heart attack that killed her in 1983.