Lampllighter from Rambla down the first 600 feet is an active landslilde and has been since the 1980's. Don't bother with it call me for a agaent who knows what they are doing!
I will answer what I can, then I will resend your question to local agents in Malibu. This question was erroneously posted in Antelope, Oregon 97001 which is why you have not heard from a Malibu agent on this thread yet.
21901 Lamplighter Lane is shown all over the web to have a "list" price of $21,500. In this case, I believe "list" price means starting bid price at auction. During an auction, prices can go many times higher than the starting bid. So the price is not "real" in the meaning that it is any where near the lot value. Many of the homes in Malibu are worth more than a million dollars. Even bare lots can be worth several hundred thousand dollars.
The auctioneer is Kennedy Wilson International and the reason for the sale is "probate" Probate means that a previous owner passed away. The property is being sold to settle the estate. While it may sell for more than the opening bid at auction, it is still likely to have a lower final sale price than a comparable property being sold by a living human being.
As to what you can build: The California Coastal Commsion http://www.coastal.ca.gov/ should be able to help you with those answers as well the Los Angeles County building department.