An ‘Installation allowance’ is a one-time payment paid to new employees of EU institutions that have had to change their place of residence. If the official is entitled to the household allowance (has a spouse or/and dependents), he/she will receive two months’ basic salaries. In all other cases the Installation allowance is equal to one basic salary.
Official cohabitation counts as family and entitles you to the allowance. In case it’s relevant to anyone, same sex couples…