1) Cooking area - This means the area used for preparing the food - large commercial gas burners, tandoor, grill etc. depending on your cuisine.
2) Work tables adjoining the cooking area for preparing and keeping the final cooking ingredients ready. This could include veggies & meat cut appropriately, sauces, spices etc.
3) A Wash area that can be used for washing the cooking utensils & accessories and another wash area for serving dishes (plates, spoons, forks etc. - cutlery, crockery). In small restaurants, the wash areas for both tend to be combined.
4) A Small Store area - This is a dry area used to store the most commonly used provisions in the kitchen. Even if the restaurant has a separate larger store, the kitchen will need a small store or atleast a few storage shelves.
5) Pantry Area - This is the area where the juices, desserts, maybe even salads and other items which do not need cooking, are prepared.
6) Food Pick-up tables - Generally, you have a small table which is used to assemble the dishes to make it easy for the server/steward to come and pick-up the items and take them for serving to the customers.
7) Preparation Area - This could be an area used to cut meats, maybe have a grinder for grinding flour etc. In small restaurants, the same work table area is used for this. This would depend on your cuisine.
8) Staff Toilet Area: This, in my opinion is critical and a very basic need you need to provide for. A number of small restaurants do not provide this and you can see the kitchen staff using the customer toilet area.
9) Staff Changing Area: A small area for the staff to change to their work uniforms/dresses and hang their clothes and keep their personal belongings.
10) Space for keeping clean cutlery and crockery: This area can be provided for adjoining the kitchen or within the customer area in the form of "Side Tables" - The next time you go to a restaurant, you can see small shelves used to store plates, spoons, forks etc. Providing a space for keeping clean water glasses is also critical as these tend to be fragile and if you do not provide for it, you will end up with lots of breakage.
11) An area where the used cutlery, crockery and glasses can be kept for washing - This is a small area where the steward / clearing boy can leave the used the plates, cutlery and glasses for the washing staff to take and clean.
If you include all of the above, you will probably need 400-600 sft even for a small restaurant kitchen. Again, the exact space requirements would depend on the concept/cuisine. So please use the above as a broad guideline for the kind of space you would need.