It depends what you want to plant. Plant what you want to eat.

Beets, carrots, short season determinate tomatoes, swiss chard, spinach and Asian greens are all space-saving crops.

Check out Vertical Garden forum for some space saving ideals.

Here is a link that might be useful: vertical garden

Your climate is great for vegetables that like cooler weather and sun - Lettuce, spinach, other greens. Maybe cabbage and broccoli. Would do one cherry tomato plant.I agree with beets, carrots, and chard.

Would ask neighbors what they grow successfully.

Beets and carrots are 12 month crops here, plant those as you wish. Neither transplant well, so plant in place. SoCal by the coast in summer is still warm -- lettuce, cabbage, broccoli and especially spinach are out until Nov, in my experience. Chard, bok choi and many greens need shade or three 90 degree days in a row will toast them in full sun, or at least cause them to bolt.

Tomatoes in coastal fog takes some doing, a community garden is a likely place for blights and everywhere I've ever seen in SoCal has powdery mildew in the neighborhood. Get resistant varieties. Slow-growing determinates are inferior to ever-growing indeterminates in fog, imo, but plenty of people here still grow them and love them. I mainly grow cherries as they can still produce when infected, but huge indeterminates are tough in a limited space community garden.

Beans, squash, cilantro, basil, are all good here in July/Aug. Melons, cukes, eggplant and peppers get progressively more difficult as summer temps drop, so too much fog or a few cool fall nights can cause the crop to fail. Aug/Sept are actually our worst months for setting out new crops -- too warm for the greens, too soon for the tubers/bulbs, too late for the heat lovers. But then watch out in Fall, when we can grow peas, all the cole crops (cabbage, broccoli, cauliflower, ...), the root crops (garlic, onions, potatoes, artichoke, asparagus,...) AND all the greens.

So no worries, you'll be able to plant a 100 sq ft area even in winter and you can grow a few crops anytime.

Here is a link that might be useful: Southern California Vegetable Planting Schedule

You have room for planting 15 plants of your choice.

How do arrive at that figure, chaman? It depends on the size of the plants. A single bush zucchini would need 4 square feet but you could get 36 little gem lettuces in the same space.

50 cabbage plants. Monotonous, but they will feed you for a year (veggie-wise).

flora, I meant 15 varieties of plants.Again this statement is debatable.but still I will try to elaborate as I will be gardening in a square plot of 10 feet x 10 feet.I will use the space to plant 3 rows of veggies. each row of 10 feet in length keeping 3 feet of distance between rows.Veggies will include zucchini ,squash , eggplant, pepper plants, tomato, a melon and/or cucumber, few beans ( not all varieties as we know there are so many ). A bottle gourd is a must for my garden which I will not dare to plant in a plot of 10x10 feet since this plant will over take all the space and cross the boundaries.There will some space for few carrots,radish, or cabbage etc.

I'm finding that hard to visualise, chaman. Why the three feet between rows? If you only have 100 sq feet isn't that a waste of space? A 3 foot strip 10 feet long takes up 30 sq feet of your 100 sq feet plot.

I agree with flora_uk.
A 5ft X 20ft would be much better & one of the reasons for raised beds is NO walking on the bed.

I think the advice you recieved from a CA member would be the most accurate. I have never grown in that situation.

I do happen to have about 100 sf. raised bed garden. This is what I have planted. 10 tomatoes some cherry some large. Underneath the tomatoes I have onions, borage, basil and canteloupe growing. The canteloupe are planted in their own 'square' but I let the vines grow under the tomatoes and trim off the very bottom of the tomato plant to give them better air circulation. I also have trellised beside the tomatoes beans and a couple cucumbers. I have 3 eggplants, 3 goosberries, 3 green peppers and a few watermelon which grow under the eggplant and gooseberries the way the canteloupe do under the tomatoes. I have 12 collards, 20 strawberry plants, 20 sq. planted with corn. On the edges of the corn I have summer and winter squash. a small spot for sweet potato. On the edge of my main planter I have winter squash but I cheat there because I let them grow out over my grass which extends the 100 sq.
Oh I also have a few radish under some of the tomatoes and such, by the time the tomatoes all get to be huge I'll harvest those and mulch the area. I practice square foot gardening combined with successive planting combined with intensive plantings. So far it has worked well except the carrots. They never got very big under my tomatoes last fall and from what I've read tomatoes actually inhibit carrots. Last year I did have a blight that spread through my tomatoes but they kept producing til the real heat hit. then I wiped the garden and replanted in the fall. I imagine Cal. is like florida you can do a spring and fall planting of tomatoes and such. I had tomatoes til Christmas. If you start your seeds in pots you can make that 100 sq. go even farther. I started my tomatoes while my carrots and turnips where still in the ground. I had 8 more weeks of growing while they got bigger. You can transplant a large portion of plants which means others can be growing while you are still waiting on germination.

I'm starting Okra tonight so that when the corn is done the heat lover can take it's place. I'll also let the sweet potato roam more, it is right beside the corn so by the time the corn is done the sweet potato will appreciate more space. As the time gets closer I'll also pre-sprout some heat loving beans (maybe, I might just do those in place).

Hello flora,
For eggplants, peppers, tomatoes etc. recommended spacing between plants in a row is 1 to 1 1/2 feet, where as spacing of 2 to 3 feet is recommended between rows.Some tropical plants like Bitter melons will occupy more than 4 to 6 feet at maturity.I keep 3 feet between rows for easiness to work for grass removal, making mounds as the plants grow and suffuicient air flow for moving and swaying of plants.I have seen gardeners keeping 2 feet of distance between rows.Going with 3 feet between rows one can plant 3 to 4 plants in a row keeping about 1/2 foot of space at the ends to plant carrots , radish, beets etc.
Your idea of 5 feet x 20 feet is far superior if one has such a plant.I had 25 feet x 50 feet plots for gardening for keeping enough spacing between rows to work easily.

I lived and gardened in San Diego on the coast for five years about ten years ago. I found that what you had on the coast was a garden year divided into thirds. There was always something I could plant, depending on the time of year. Too cold for tomatoes in the winter, never really too hot for much of anything. I grew peanuts for the fun of it. Broccoli and chard never died and got as big as you let it. Eventually every old plant got some kind of bugs. This was in a community garden, and yes, disease and bugs were issues, so I would make disease resistant cultivars a priority, and leaving enough space between plants to get the aid of the wind in reducing the disease pressure. Ground squirrels were a problem in that garden.

So-decide what you like to eat, and what is expensive to buy, and plant those things. What is the soil like? How fertile your soil is will determine how much you can expect to get from your garden. Work on building up your bit of soil, and it will reward you. After five years I left a pretty good bunch of soil for the next person. In my 100 sq ft garden, I would "plant soil", and the vegs will take care of themselves.