What Is a Co-Signer on a Mortgage?

Short answer

A co-signer signs the mortgage note and is jointly liable for 100% of the debt but has no ownership interest on title. Fannie Mae treats co-signers, guarantors and non-occupant borrowers under the same rules: up to 95% LTV on a DU-approved loan or 90% if manually underwritten when their income is used (as of November 2026).

Definition

Per Fannie Mae, a co-signer or guarantor signs the note and has joint liability with the borrower but does not have an ownership interest in the property as shown on title.

Worked example

Example only, not a quote: a nurse earns $5,000 a month. Her new house payment would be $2,300 and her car and card payments $500, so her DTI alone is 56% ($2,800 / $5,000). Her brother co-signs with $7,000 a month of income and $1,200 a month of his own debts. Combined: $4,000 of debt on $12,000 of income, a 33% DTI. On a manually underwritten loan, Fannie Mae also requires her DTI using only her income to be 43% or less, so this file would need DU approval, subject to DU findings and credit approval. The brother's credit report will show the whole loan.

Co-signer vs co-borrower vs non-occupant co-borrower

• Co-signer / guarantor: signs the note, fully liable, not on title.
• Co-borrower: signs the note and is normally on title as an owner, often living in the home.
• Non-occupant co-borrower: signs the note and may or may not be on title, but does not live in the home.

In every case the person's income and debts count, and the payment counts in their DTI on future loans.

Fannie Mae and FHA rules

Per Fannie Mae Selling Guide B2-2-04 (as of November 2026), co-signers are allowed on purchases, limited cash-out and cash-out refinances. They cannot have an interest in the sale, so not the seller, builder or real estate agent. On manually underwritten loans, the occupying borrower must usually put in the first 5% of the down payment from their own funds (exceptions include 80% LTV or less). FHA loans also allow co-signers and non-occupying co-borrowers under HUD Handbook 4000.1, with their own LTV rules, so we confirm the HUD guideline for your file. Rates vary daily; call for today's pricing.

In South Florida

In Miami many families buy together, and family members often help each other qualify. A co-signer is not on title, so they get no ownership and no Florida homestead exemption, yet they owe the full debt. If the helper wants ownership, they may be better as a co-borrower on title; ask your title company or attorney how title should be held.