The good news is, Bay Area winters are extremely mild. Mild enough that a bun could probably survive one just fine without any extra assistance. The coldest it gets on average is around 50°F. I know this because my boyfriend is from Santa Cruz and lived in SF for 8 years, and I’ve also got family who live in SF. The shock for him to get used to East Coast winters was pretty bad, haha!

Anyway, if you are worried about it or if the bun’s area gets a bit drafty, there’s a few things you can do. The easiest and most drastic one is to buy a small space heater. You can get one for very little online or even in cheap stores. You don’t need anything over the top, just something to raise the tempetature a bit. Place this in the corner of the room farthest from the bun’s area. If she’s free range, you could easily block off access to the cords and to the heater by placing a pet gate, x-pen, or NIC cubes in front of it. Some heaters are small enough that you can put them on a shelf, so that keeps the unit itself at least farther out of reach.

Have you tried SnuggleSafe units?

Whatever you try that holds or produces heat, you can make inaccesible by blocking them off physically. This could be SnuggleSafes, those rice / oatmeal heat pouches, or anything that will hold heat.

What’s her current area like? Does she have a whole room to free roam or is she in a confined space of some sort?

You can put a blanket over her cage or part of her cage.

Rabbits actually do very well in chilly weather and 50 F is not a problem for a bunny, especially not if the cooling down is gradual, as it would be in your case. They just get thicker fur. We have VERY severe laws about keeping animals in Sweden and here it’s actually allowed to keep rabbits in unheated spaces if the cages are elevated off of the ground and sheltered from wind, rain and snow and you just give them water twice a day during the period when the water in their bowls freezes solid. People do that all the time up in the north where it gets really cold. I don’t know of any other animal that you may keep this way legally, hens f ex must have heating. Guinea pigs and hamsters are not allowed to live outside at all.

You can perhaps elevate her cage some 4 inches or so off the floor and if you can, put styrofoam under it for insulation. A rather fat block of styrofoam would be ideal. Otherwise a stack of newspaper has insulating properties. You can allow her some extra pellets during the cold season, keeping warm requires extra energy.

Bunnies do a lot less well in hot climates so I think your bunny will be quite fine in the area where you’re living now.

Edit: if she’s only confined at night you can get her a wooden box that she can lie on top of during the days, if she likes to lie outside her cage. Just getting a few inches up from the floor is more warm, plus bunnies do tend to like to sit/lie on top of boxes etc anyway. (In Sweden we actually have a law saying bunnies’ cages must be provided with an elevated resting-place, no matter where the cage is kept. So you see we are very serious about pet wellfare here =) ) 

I have a sheet over an 1/8th of Theodore’s pen just to give him some cover. He seems to like to hang out in that area of the pen, but I don’t think it does much for the temperature.

I think bam‘s ideas are good ones. If only we had the same animal laws as Sweden…

But ultimately, you don’t have to worry too much.