It's important to remember that there are ways to help bees with or without a garden. For even the smallest of spaces, a window box could have a selection of herbs for kitchen use, whose flowers could also feed the bees.
Preferably, bees like a large area (or clump) of the same species of flower. These patches allow them to save travelling time and fuel, as they can source a large quantity of nectar or pollen in one go.
Bees can return to their hive and 'tell' other bees where the best forage is, so a larger clump also makes giving directions easier. For this reason, a large area of blue lavender, for example, would be a lot easier to find than a single bergamot in a mixed flower bed.
If you don’t have a lot of space on your doorstep or window boxes, think about concentrating on one type of plant that has a more extended flowering period. For instance, lavender, catmint, phacelia, borage (great for your summer cocktails) or salvias would all be suitable.
Passiflora has always been popular with my bees; the flowers often have three bees on them at any one time. A great hardy climber, one idea could be to cover your wall with passionflowers (and bees) from June to November.
If you have a flat roof, covering it in sedum would also offer a fabulous plate of forage.
If you have no room for flowers but still want to help, then maybe join your local community group or council. By influencing their planting choices on public spaces and verges, you will be able to help the bees as much as a well-planted garden. As far as bees are concerned, ‘every little helps’!