“The house is my girlfriend’s parent’s house and is to be used as a big chunk of their retirement when that time comes”
OK, are your future in-laws planning on selling the house to cash out for retirement?
If so, how bout you guys make a deal with them that you want the house as an inheritance and you will give them an advance for the value.
Could you afford the house at the time of their demise? Is this a neighborhood you would like to raise your family?
What I’m getting at is that your future wife and you can inherit the Prop 13 property taxes your in-laws are paying. If your in-laws have owned this home for a long time (and the value doesn’t crash below what they paid for it) you could set yourself up to save the proverbial sh**-load of $$ in future property taxes.
And of course as soon as they lay eyes on their grandchildren they will want to give you two a smoking deal.