I have several parcels of land for sale - it is up to you to do your own investigations and I have the resources available for you if you would like to contact me. Don't rely on anyone's advice, do the research and find out for yourself.
I've done full interior remodels on two homes. I've been I finance for 10 years, I've dealt with la building and safety , I've been an appraiser for 3 years, and I've done my first retaining wall. Construction is sometimes tricky. My advise to you, if you start with a foundation contractor and soils and geo engineer, that's your best bet before you buy the lot or hold it up contingent on your investigation. Buy the flat lots or flat as possible , stay away from hillside .
I have to respectfully disagree with statements about Topanga being "built out". If you are building a smaller home with one bedroom and limited foot print, then you will have more choices for land. Placing the home closer to road frontage also helps keep cost down.
It will be difficult to find such a situation here in Topanga or Malibu. Most lots that are priced at $70k or less are either un-buildable or will be very difficult and very costly to develop.
The lowest priced and easiest to build lots in Topanga are in Top O' Topanga which is a manufactured home community. The lots are zoned for manufactured homes and the permitting process to have one installed is MUCH easier than developing raw land in the Topanga.
The down side is that you're in a neighborhood with many other mfd. homes very nearby.
The upside is you're in Topanga and can see the stars and hear the owls hooting and the coyotes yipping. Prices for newer homes in this area (mfd by Silvercrest and the like) are currently in the mid $300k range. Older homes are going for under $200k.
In fact, an older home, probably circa 1970, was recently purchased for about $175,000 and is being replaced with a new home. All of this has taken place within a couple of months - the building process in Top O' Topanga really is quick compared to elsewhere in the Santa Monica Mountains.
I hope this helps! Please let me know if you want to check this neighborhood out!
Prudential Malibu Realty - Topanga