Changes to housing benefit will affect 350 people living in council houses across Harrow.
The new tax on bedrooms will apply to working age tenants deemed to have surplus bedrooms and will come into effect on April 1.
There now only need to be one room for each adult or couple, one room for every two children under age 10 regardless of gender and one room for every two children under 16 if they are the same gender.
Housing benefit will be reduced by of 14 per cent for one extra bedroom and 25 per cent for two or more extra bedrooms.
Harrow Borough council is offering advice and support to tenants about the changes.