Income for SSI
If you are collecting or applying for SSI, your roommate may, in some circumstances, have an effect on your eligibility for benefits. If you own the property, for example, and you rent out a room in your own home to a roommate, any rental payments your roommate makes to you are taxable as income. If you are both renting, however, there is generally no problem with SSI, since the rental income is taxable to the landlord, not to you. It is not generally considered income to you. However, if you are paying less than the market rate for rent, and someone else is paying your share for you, this may count against you.
Income for SSDI
The principle for SSI remains true for SSDI, though the way the calculation is implemented is different. SSDI does not prohibit you from having a substantial income. But the government reduces benefits if your income, excluding exemptions, rises beyond 80 percent of your predisability income.
The Social Security Administration differentiates between family income and household income when calculating SSI eligibility. This means that if you have a roommate who is earning an income that would put you over the SSI income threshhold and that roommate is not related to you, the SSA would not count that income against you. If your roommate is a relative, though, the SSA may consider the income to be family assistance paid on your behalf. This would potentially count against you for eligibility purposes.
To avoid any problems or disputes concerning contributions from a paying roommate, you may consider adding your roommate to the lease, so that your roommate can pay the rent directly. That way, your roommate's money never even passes through your hands.