Would it be difficult to find lodging without reservations on the coastal drive from LA to SF California during the summer? Landing in LA and taking 2 weeks to amble our way to SF where we would end our trip. I would probably make hotel reservations in the LA area and in SF only. Good idea or not. 2 adults and a teen.

Two weeks on the road or two weeks including days in LA and SF? The main place I would book ahead is anywhere around the Hearst Castle if that's on your agenda.

Sounds great with a whole 2 weeks to savor almost the entire coastline.
There might be a problem getting lodging on the weekends, Friday and Saturday nites, so if you can plan ahead at all, do so.
California has an excellent tourism website at:

http://www.visitcalifornia.com/state/tourism/tour_homepage.jsp

I believe you can also order a tourism packet of info from this website, too.
Do order it ASAP.
Also, I assume you'll be touring Hearst Castle; be sure to reserve ahead for the tours you want...summer is very busy. Its website should be something like:
www.hearst.org
If that's not correct, use:
www.google.com
an excellent search engine.

It would probably be VERY difficult in places like Cambria, Big Sur/Carmel, Capitola. A little easier in Santa Barbara, Santa Cruz or Monterey.

Weekends will require reservations everywhere on your itinerary.

We often have done this trip. If you aren't picky, and can tolerate a motel six type place, you should be able to just wing it; Cambria/Hearst Castle being the exception.

big sur only has a handful of places to stay, i'd definitely book that one in advance. i'm assuming in 2 weeks time, you'll spend several days at a few points of interest. the actual drive can easily be done with 1 overnight.