To change who owns your house, you have to change the deed. That's just as true if you want to share the title with your partner as it is when you sell to a new owner. The difference is that instead of giving up the title, you transfer it from yourself to yourself, but with someone else added.
To add another person to a house deed you must make out a new deed listing yourself as the grantor and yourself and the individual as the grantees.
Changing the Deed
The basics of adding someone new to your deed are the same in every state. You, as the "grantor" -- the property owner -- make out a deed conveying title to two "grantees," yourself and your partner. If you want to transfer title to a friend or your child, the same principle applies. You sign the deed as grantor, have it notarized and file it with the county's registry of deeds. Some states require extra steps, such as having witnesses or the grantees sign as well.
Using a Quitclaim Deed
The two main types of deeds are warranty and quitclaim. Warranty deeds are standard when buying a house, as they make the seller liable for title problems. Quitclaim deeds exempt you from liability for any title problems that crop up after the deed is filed. That makes them risky for homebuyers, but if you're adding a co-owner, they're a good choice. As you know and presumably trust yourself not to give yourself a phony title, using a quitclaim should be perfectly safe.
When you make out the new deed, put in writing how you and your co-owner want to share the title. If you want to share everything equally, joint tenancy with right of survivorship gives you each a 50 percent share; if one of you dies, ownership passes automatically to the other. With tenancy in common, you can have unequal shares -- if you're still paying the mortgage, you might want 75 percent ownership, say -- and your share passes to whoever is named in your will.
Add Your Partner's Name to the Mortgage
If your name is on the mortgage, transferring title with a quitclaim deed doesn't affect your obligation to pay or add your partner's name to the mortgage. If your mortgage has a due-on-sale clause, however, the bank might claim that transferring the title violates that clause and you have to pay your debt immediately. It's safer to get the lender's approval in writing before changing the deed. If the lender doesn't approve, you may be stuck until you can refinance.