A policy brief is a kind of report, aimed at government, local authority, or third sector policy makers. In a policy brief, you are aiming to influence policy making, by taking a persuasive, evidence-based stance.
Look at the section on the capabilities approach (p16). Choose a group with particular housing needs. For instance, this might be:
People without access to shelter.
Families in temporary accommodation.
Care leavers.
Asylum seekers.
Any other group you choose provided they have specific housing needs.