I'm not an attorney or a tax advisor, the. The following is NOT to be considered advice, just general information.
In the US, "putting your name on the deed" would mean making you a co-owner. Absent any other legal agreement between you (e.g. a contract stating each of you owns 50% of the house), both of you would then be considered to own 100% of the house, jointly and severally:
- neither of you would be able to perform any transaction on the house without permission (or possibly, physical presence) of the other
- this leads to potential hassles if the partnership is dissolved (including via death)
- both of you would be 100% liable for any liability caused by the house (e.g. real estate taxes; injuries to visitors or tradesmen performing work; damage to nearby houses, trees, or cars)
- both of you would owe capital gains taxes after selling the house at a gain
In addition, the IRS would almost certainly interpret the creation of your ownership interest as a gift from your partner to you, making them liable for gift tax. The gift tax could be postponed by filing a gift tax return, which would reduce partner's lifetime combined gift/estate tax exemption. And if you sought to get rid of your ownership interest by giving it to your partner, it would again be a taxable gift, with the tax (or loss of estate tax exemption) accruing to you.
However, it is likely that this is all moot because of the mortgage on the house. Any change to the deed would have to be (a) approved by the mortgage holder and (if so approved) executed by a title company/registered closing agent or similar (depending on the laws of your state). In my similar case, the mortgage holder refused to add or remove any names from the deed unless I refinanced (at a higher rate, naturally) making the new partners jointly liable for the mortgage. We also had to pay an additional title fee to change the deed.