Adude's advice is excellent. You definietly need to get the advice of a tax professional, and perhaps a local estate attorney as well. There are issues because it is your parent's home.

I do repsectfully disagree with my friend Gresch on one point, however. I think a BMW board is an excellent place to get advice on a complicated matter such as this. You can find (and have found) many smart, generous professionals here who will take the time to offer you an answer full of many useful ideas and much insight. By asking the question here, you now know to ask other questions. So from that point of view, this is the FIRST place to ask questions like yours.

Either way, good luck, and to echo the point made by many others, DO NOT buy a house with your girlfriend, especially your parent's home.

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-'13 E93 M3 Cab, Interlagos Blue, Carbon Black leather, DCT, all options, and fun, endless fun!
-'13 X5 35i M-Sport, Carbon Black & Cinnamon, Dinan Stage 2!
-'15 Toyota Sienna Limited Premium AWD, Silver & Grey, Nav & BluRay, and less lame than the 2011 it replaced
-'07 Mini Cab S, Laser Blue, 6sp, endless fun that ended (sold)
-'04 X5 4.4i, loaded, loved, modded and sold