A bee's needs
Bees need two key resources to successfully reproduce: food and nesting space. Food for bees comes from flowers (pollen for the larvae, nectar for the adults), so the more bee-friendly flowers you can grow, over as much of the year as possible, the better for bees.
Nesting space can be a little more complicated. There are ~ 250 species of bee in the UK, and many of those have very different nesting preferences. These include warm sunny slopes, clean hollow sticks, crumbling mortar, pesticide-free lawns, abandoned mouse holes and hollow trees - there are a lot of places that different bees might find themselves comfortable! Unfortunately, our tidy gardens often lack suitable sites, but we can make effort to provide places for our pollinators.
Some bees nest well in holes in wood, which can form the basis of a 'bee house'.
Most common species prefer dark and dry cavities, often nesting underground. Abandoned rodent holes or underneath sheds are likely places (also check your compost bins!). Some species nest above ground, using moss and thick grass.
Although there are a range of bumblebee nest boxes for sale, there are cheaper ways to provide nesting habitat for bumblebees. Upside down, half-buried teracotta plant pots (hole up) with a bit of old hamster bedding inside may attract bumblebees.
On a larger scale (left), old pallets, loft insulation and some wooden boarding can make the perfect place for bumblebee colonies.
The majority of these bees nest individually, with one female laying and provisioning (rather than having a Queen and workers like bumblebees or honeybees do). Many of them do not mind nesting nearby to others though, so one 'bee hotel' can have several females that live there, just not together.
They generally prefer holes in mortar, hollowed out plant stems, dead-wood or holes dug into bare soil. Providing 'bee hotels' (bought or homemade; an impressive example shown right) is an effective way to help solitary bees nest in your garden.
Bee hotels can also provide nesting sites for other insects (sometimes better than for bees, so don't be discouraged), so are great for increasing biodiversity within the garden.