Harold's house
"I suppose you think this is very funny, Harold."
The Chasen Mansion is the house where Harold lives with his domineering mother. It's where he stages his many fake suicides; it's where his mother makes her various pronouncements about Harold's future; it's where Harold's three computer dating service dates take place; and it's where Harold's mother gives him a sports car and where Harold dons a welding mask to remake it into a miniature hearse.
The mansion is located at 10 Stacey Court in Hillsborough, a dead end street in the hills above Burlingame. The exterior of the house is much the same as it was 42 years ago, although SUVs have replaced hearses:
This mansion was (and perhaps still is) known as the Rose Court Mansion. Someday I will get up the nerve to ring the doorbell and ask for a tour of the interior.
Here's the mansion's mailbox; you can see the number 10. Some sites erroneously report the address as 4 Stacey Court.
And here's the view of the mansion in Google Maps. You can see the pool where Harold stages one of his suicides:
Fun fact: Remember the scene were Harold's third date accidentally kills herself, and Harold's mother appears at the door accompanied by the butler, and she tries to place a glass of lemonade on the butler's tray and misses, dropping it on to the floor? The butler in that scene is played by the mansion's real butler. In the script, he was the one who was supposed to drop the lemonade on the floor, but he refused. So they had to rewrite the scene to have Harold's mother drop it.