How to divide a mortgage and living area fairly?
I suggest that you first decide on what %'s of the home value you each have a legal claim to. Then split the mortgage using the same %'s. Then, if someone feels their % is slightly higher, they are compensated because they 'own' a correspondingly higher share of the house. Use the same %'s for downpayments (which may mean that an 'adjustment' payment might be required to bring your initial cash outlay from 70/30 into the %'s that you agree to). Tenant income gets split the same way. Utilities are a bit more difficult - as heating depends more on square feet, but water and hydro depend more on how many people are there. You can try to be really precise about working out the %'s, or just keep it simple by using the same %'s as the mortgage. In my opinion, since she will live in one apartment, as will you and your husband, the simplest method is to divide the ratio exactly the same as the area for your living space. If it's 40/60, she puts 40% down, you put 60%. And you split expenses the same.  The tenant income can be applied to the house expenses, as it's no different than giving her 40% and you keep 60%.  No matter how well you get along, it's easy for someone to feel a split of expenses isn't fair unless it's discussed and agreed up front. I think what you have here is actually TWO agreements with your sister, and explicitly splitting it into two agreements will bring some clarity. The first is ownership of and responsibility for the building. The second is each of your personal use of a unit.  Here's what you do: Treat ownership as if you're not living there. Split the down payment, the monthly mortgage, taxes and insurance, responsibility for cost of maintenance, etc. as well as the ownership and benefit of the building 70%/30%. Put all that in a contract. Treat it like a business. Second, lease those units to yourselves as if you were tenants. And yes, I means even with leases. This clarifies your responsibilities in a tenant capacity. More to the point, each of you pays rent at the going rate for the unit you occupy.   If rent from all three units equals the monthly expenses, nothing more needs to be done. If they're more than the monthly expenses, then each of you receives that as business income on that 70%/30% breakdown. If those three rents are less than the monthly expenses, then each of you are required to make up the difference, again at 70%/30%.  Note: if any of those expenses are utilities, then they should be apportioned via the rent -- just as you would if you'd rented out the whole building to strangers.  2nd note: all that can be done with ledger entries, rather than moving money around, first as rent, then as expense payments, then as payouts. But, I think it will benefit all of you to explicitly pay rent at first, to really clarify your dual relationship as joint owners and as tenants.  Final note: I think this is a stickier situation than you may think it is. Familial relationships have been destroyed both by going into business together, and by renting to family members. You're doing both, and mixing the two to boot. I'm not saying it will destroy your relationship, but that there's a solid risk there. Relationship destruction comes from assumptions and vague verbal agreements. Therefor, for the sake of all of you, put everything in writing. A clear contract for the business side, and clear leases for the tenant side. It's not about trust -- it's about understood communication and positive agreement on all important points.
I can answer Scenario #3. If you are purchasing a property with buy-to-let intentions […] can you use the   rental income exclusively to fund the mortgage repayments? Yes – this is exactly how buy-to-let mortgage applications are evaluated. Lenders generally expect you to fund the mortgage payments with rent. They look for the anticipated monthly rent income to cover a minimum of 125% of the monthly mortgage payment. This is to make sure you can allow for vacant periods, maintenance, compliance with rules and regulations, and still be in profit (i.e. generate a positive yield on your investment). However, buy-to-let (BTL) mortgage lenders also generally expect you to own your own home to begin with. It's up to them, but rare is the lender who will provide a buy-to-let mortgage to a non-owner-occupier. This is because of point 2 above. The lender doesn't want you to end up living in the property because then you'll need to repay the loan capital, since you'll always need somewhere to live. This makes the economics of BTL unfavourable. They look at your application as a business proposal: quite different to a residential mortgage application, which is what your question seems to be addressing. Bottom line: You're right about scenario #3 but it sounds like you're trying to afford a home first, whereas BTL is best viewed as an investment for someone who already has their main residence under ownership (mortgaged or otherwise). As for Scenarios #1 and #2 I can't offer first hand answers but I think Aakash M. and Steve Melnikoff have covered it. When over the long term housing costs in a area rise faster than wages rise, the demographic of who lives in the area changes. The size and income parameters change. A region that was full of young singles is now populated with couples with adult children, that means that the businesses and amenities have to change. At a national level it isn't sustainable unless other items change. The portion of monthly income that can be safely allocated to housing would have to change. One adjustment could be the the lengthening of home loan periods, thus dropping the monthly payment.  This has been seen with car loans, over the last few decades the length of loans has increased. In interesting related event could be the change in deduction of mortgage interest and property tax. If this was to change abruptly, there could be an abrupt change the estimated value of housing, because the calculus of affordability would change. "BLUF:  Your question is subjective and as such, the answer varies from person to person.   The rent you ""should be"" paying is the tricky part.  Minimum is whatever is the least you can find.  Maximum is the most you can afford.  To be financially responsible, you would live as close to that minimum as you can bear.  However, this can cause stress if you are trying to subsist on lower than you can actually bear. You have certain expenses that are required to survive.  Housing, clothing, food.  These are your needs.  Everything else is convenience or luxury. Best way to develop a budget is to list the categories of everything you have spent over several months.  Figure out a monthly amount for each one.  Divide them up into groups of things you need, things that make life bearable, and things you can do without.   Then start with your income and figure out how much you bring home each pay period.  When you get paid, allocate the money to your needs until they're all covered.  If there's anything left, fund the second group, then on to the third.  Continue tracking your spending and adjust where you allocate the money.  After 2-3 months, you'll start having a decent idea on how much you actually spend on each category.  You'll probably find areas where you're spending a lot more than you realize.   The method I've just described is the one advised by ""You Need a Budget"" software, but you don't need the software to use the method.  Though it does help. Also, if there's any debt, that goes in with the 'needs' because we all need to pay our bills." How about a slightly different approach. Invest in a duplex or trip/quadplex. Live in one unit, rent out the rest. Chances are you'll end up either paying nothing in total, or even making money as your tenants pay your bills for you. Depends a lot on your area and your willingness to deal with the crap of tenants, but have a look into it. You'll be surprised what you can buy in your area and the types of people you might end up living with you... It is great that you came up with a plan to own a rental home, free and clear, and also move up in home.  It is also really good of you to recognize that curtailing spending has a profound effect on your net worth, many people fail to acknowledge that factoid and prefer to instead blame things outside their control.  Good work there.  Here are some items of your plan that I have comments on. 11mo by aggressively curtailing elective spending How does your spouse feel about this?  They have to be on board, but it is such a short time frame this is very doable. cashing out all corporate stock, This will probably trigger capital gains.  You have to be prepared to pay the tax man, but this is a good source of cash for your plan.  You also have to have an additional amount that will likely be due next April 15th. redirecting all contributions to my current non-matched R401(k) This is fine as well because of the short time frame. withdrawing the principal from a Roth IRA This I kind of hate.  We are so limited in money that we can put into tax favored plans, that taking money out bothers me.  Also it is that much more difficult to save in a ROTH because of the sting of taxes.  I would not do this, but would favor instead to take a few extra months to make your plan happen. buy home #2 How are you going to have a down payment for home #2?  Is your intention to pay off home and save a while, then purchase home #2?  I would do anything to avoid PMI.   Besides I would take some time to live in a paid for house. Overall I would grade your plan a B.  If take a bit longer, and remove the withdrawing from the ROTH, it then becomes an A-.  With a good explanation of how you come up with the down payment for house 2, you could easily move to an A+. First, i think you're doing awesomely for your age. Here's what i'd do in your situation (disclaimer: These are just my personal opinions from experience with my own finances.): I'd do all those things and partition the money so that i ensure i do them all. That may mean not dollar cost averaging monthly but rather quarterly to keep fees-percentages down, but i think that's reasonable for your age. Something i don't think you should overlook with regard to your mortgage is the freedom afforded you by paying off a home. It provides you with the freedom to be out of work, between work, or take an extended leave without the fear of how to pay your bills, the mortgage tending to be a significant percentage of the monthly bills. If that's not something you've considered, not a concern, or not something you care about, then paying off your home probably isn't a priority so I'd drop that step and put more money into investments. Well, it sounds like you have two options: 1) Continue to jointly own the house. 2) Compensate her for her equity and get the title transferred.  I hate to tell you this, but she is entitled to half of the equity regardless of how much she paid into it. That said, she is still on the hook equally for the loan amount, but it won't do you any good if she is not willing to pay. Also, option 2 probably isn't a good deal for your co-signer as she would still be liable for the entire loan loan (just as you are) regardless of the title. You're worried about your tenant. That just means you're a nice guy, and it's ok to be nice. At the same time, you can't be expected to lose money on the property or charge well below market on the rent. My suggestions: You know what? She'll totally understand. You've been super nice in keeping the rent low for so many years, and she's been a great tenant, too. At a certain point, inflation kicks in and you have to raise the rent. She'll get that. If she can find a cheaper place, that's a win for both of you. Help her move if you want to be extra nice. Then decide if you want to sell the place or raise the rent. Either option is fine. Listen to your wife. That's just general advice.