Fair Value of a monthly payment given two Bank Payment structures
There is one basic principle to apply here:  to compare money paid at different times, all the amounts must be compounded or discounted to the same point in time. In this case, the moment of the initial  $225,000 loan is convenient. At that moment, you get $225,000 You then make 30 payments on the 40% mortgage.  The amount of these payments has to be calculated;  they're paying off a $90,000 mortgage with 30 monthly payments at a monthly rate of 0.5% Finally, you make 30 payments of an amount X, starting one month after the 40% mortgage ends. So far we've just listed the amount and time of all the payments back and forth. A time-line type diagram is a huge help here. Finally, use compound interest and annuity formulas to bring all the payments to the starting point, using an interest rate of 1% a month! Equate money in with money out and solve for X
The ownership of the house depends on what the original deed transferring title at the time of purchase says and how this ownership is listed in government records where the title transfer deed is registered. Hopefully the  two records are consistent.  In legal systems that descended from British common law (including the US), the two most common forms of ownership are tenancy in common meaning that, unless otherwise specified in the title deed, each of the owners has an equal share in the entire property, and can sell or bequeath his/her share without requiring the approval  of the others, and joint tenancy with right of survivorship meaning  that all owners have equal share, and if one owner  dies, the survivors form a new JTWROS. Spouses generally own property,  especially the home, in a special kind of JTWROS called tenancy by the entirety. On the other hand, the rule is that unless explicitly specified otherwise, tenancy in common with equal shares is how the owners hold the property. Other countries may have different default assumptions, and/or have multiple other forms of ownership (see e.g. here for the intricate rules applicable in India). Mortgages are a different issue. Most mortgages state that the mortgagees are jointly and severally liable for the mortgage  payments meaning that the mortgage holder does not care who makes the payment but only that the mortgage payment is made in full. If one owner refuses to pay his share, the others cannot send in their shares of the mortgage payment due and tell the bank to sue the recalcitrant co-owner for his share of the payment: everybody is liable (and can be sued) for the unpaid amount, and if the bank forecloses, everybody's share in the property is seized, not just the share owned by the recalcitrant person.  It is, of course, possible to for different co-owners to have separate mortgages for their individual shares, but the legalities (including questions such as whose lien is primary and whose secondary) are complicated. With regard to who paid what over the years of ownership, it does not matter as far as the ownership is concerned. If it is a tenancy in common with equal shares, the fact that the various owners paid the bills (mortgage payments, property taxes, repairs and maintenance) in unequal amounts does not  change the ownership of the property unless a new deed is recorded with the new percentages.  Now, the co-owners may decide among themselves as a matter of fairness that any money realized from a sale of the property should be divided up in accordance with the proportion that each contributed during the ownership, but that is a different issue. If I were a buyer of property titled as tenancy in common, I (or the bank who is lending me money to make the purchase) would issue separate checks to each co-seller in proportion to the percentages listed on the deed of ownership, and let them worry about whether they should transfer money among themselves to make it equitable. (Careful here! Gift taxes might  well be due if large sums of money change hands). "There are two ways that mortgages are sold: The loan is collateralized and sold to investors. This allows the bank to free up money for more loans. Of course sometime the loan may be treated like in the game of hot potato nobody want s to be holding a shaky loan when it goes into default. The second way that a loan is sold is through the servicing of the loan. This is the company or bank that collects your monthly payments, and handles the disbursement of escrow funds. Some banks lenders never sell servicing, others never do the servicing themselves. Once the servicing is sold the first time there is no telling how many times it will be sold.  The servicing of the loan is separate from the collateralization of the loan. When you applied for the loan you should have been given a Servicing Disclosure Statement Servicing Disclosure Statement. RESPA requires the lender or mortgage   broker to tell you in writing, when you apply for a loan or within the   next three business days, whether it expects that someone else will be   servicing your loan (collecting your payments). The language is set by the US government: [We may assign, sell, or transfer the servicing of your loan while the   loan is outstanding.] [or] [We do not service mortgage loans of the   type for which you applied. We intend to assign, sell, or transfer the   servicing of your mortgage loan before the first payment is due.] [or]   [The loan for which you have applied will be serviced at this   financial institution and we do not intend to sell, transfer, or   assign the servicing of the loan.] [INSTRUCTIONS TO PREPARER: Insert   the date and select the appropriate language under ""Servicing Transfer   Information."" The model format may be annotated with further   information that clarifies or enhances the model language.]" "Not to state the obvious, but whenever an investment is being made, the ""nuts and bolts"" is your return on investment.  Analyzing the rate of return on an investment is the primary factor in any decision.  Ideally, once the actual mechanics of investment and side ""benefits"" are factored out, the goal is to be able to analyze the pure financial return. Usually the biggest problem faced in analyzing various investments is comparing the Present Value of an investment to a series of payments that may be made or received in the future.  When considering the purchase of a large equity, for example, you might be looking at what series of payments are required to purchase the asset.  You can also reverse this and ask, ""What amount of money is equivalent to this series of payments?"" Ultimately, the Present Value of an Annuity is the way to make these comparisons equal.   Fundamentally, the Present Value of an annuity is an amount of money that should, in theory, be equivalent to a series of payments.  There is, for example, technically no difference between $1064.94 today and $100 a month for a year, at an interest rate of 1% per month.  Grant you, most people would be happier with the money now, but that is what interest does - it compensates you for waiting on your money.   You can fire up a spreadsheet and calculate the Present Value as long as you have the monthly payment, interest rate, and number of periods.  Alternatively, you can calculate any one of those missing four variables - and the key is usually to understand what that rate would be in order to compare the investments. Finally, the taxable implication is really just an adjustment to the rate of return. Imagine the following three scenarios: (Obviously the rates are fictional - the goal is to show they are the same). Scenarios 1 & 2 are really just two sides of the same coin.  Using the Future Value formula in Excel = FV(0.5%, 12, -100), you get $1233.56.  In scenario 1, you would have $1233.56 in your bank account.  In scenario 2, your bank would have $1233.56 from you, and you would have $100 less debt per month.  They are equivalent transactions.   Scenario 3 is really just a variation on scenario 2, localized to the United States.  Because the interest is tax deductible, however, the rate of 6% isn't really accurate.  Assuming you had a 25% tax bracket, you'd actually be getting back one quarter of your interest.  Put another way, 7.5% mortgage interest costs you as much as 6% credit card debt. This is how you compare apples and organges - just turn everything into an annuity or a lump sum, using Present Value calculations.   Finally, quick rule of thumb - if you owe taxes in both Canada and the US, your Canadian taxes are probably higher than your American ones.  As such, any tax incentives will be concomitantly higher.  If you only can only use Canadian tax incentives, then look to those incentives, other things being equal." An amortization schedule is often used to produce identical payments for the term (repayment period) of a loan, resulting in the principal being paid off and the debt retired at the end of the loan. This is in contrast to an interest only, or balloon loan. These loans require little or no payment against the balance of the loan, requiring the loan to be paid indefinitely if there is no term, or requiring the loan to be entirely paid off from cash or a new loan at the end of the term. A basic amortization formula can be derived from the compound interest formula:   This formula comes from the Wikipedia article on amortization. The basics of the formula are the periodic payment amount, A (your monthly payment), can be determined by the principal loan, P, the rate, r, and the number of payments, n.  Lenders lend money to make a profit on the interest. They'd like to get back all the money they lent out. Amortization schedules are popular because the fixed low payments make it easier for borrowers to pay the loan off eventually. They also tend to be very profitable for lenders, especially at the start of the term, because they make a lot of profit on interest, just like the start of your mortgage. The principal of a mortgage has more meaning than the principal of a revolving debt credit card. The mortgage principal is fixed at the start, and represents the value of the collateral property that is your home. You could consider the amount of principal paid to be the percentage of your home that you actually own (as part of your net worth calculation).  A credit card has a new balance each month depending on how much you charge and how much you pay off. Principal has less meaning in this case, because there is no collateral to compare against, and the balance will change monthly. In this case, the meaning of the amortization schedule on your credit card is how long it will take you to pay off the balance if you stop charging and pay at the proscribed payment level over the term described. Given the high interest rate on credit cards, you may end up paying twice as much for goods in the long run if you follow your lenders schedule. Amortizing loans are common for consumer loans, unless a borrower is seeking out the lowest possible monthly payment. Lenders recognize that people will eventually die, and want to be paid off before that happens. Balloon and interest only bonds and loans are more commonly issued by businesses and governments who are (hopefully) investing in capital improvements that will pay off in the long run. Thousands of people and businesses have gone bankrupt in this financial crisis because their interest only loans reached term, and no one was willing to lend them money anymore to replace their existing loan. "I too am a full-monthly-statement-balance payer and I received a balance transfer offer from my credit-card company. This one was quite different from many others that I have read about on this forum. I could do a balance transfer for any amount up to $X from another credit card, or use the enclosed ""checks"" to pay  some other (non-credit-card) bills, and I would not have to pay any interest for 12 months on the amount thus borrowed.  But, There would be a 2% service charge on the amount I was borrowing. This amount would be billed on the next monthly statement, and it would have to be paid in full by the due date of that month's payment, that is, within the 25-day grace period allowed for payment of monthly statements. Else, interest would start being charged on the unpaid part of the service charge at the usual humongous rate of H% per month. If I had not paid the previous month's balance in full, I would be charged interest at H% per month on the service charge starting from Day One; no free ride till the due date of the next month's statement. Of course, the balance carried over from last month would also be charged interest at H%. If I had paid last month's bill in full, but there were any other charges (purchases) during the current month, then  unless the entire amount due, this month's purchases plus service charge and that ""interest-free-for-twelve-months loan""  balance was paid off within the 25-day grace period, my purchases would be deemed unpaid and would start being charged interest. In short, the only way to avoid paying interest on the amount borrowed was to start with a card showing a $0 balance due on the previous month's statement, not make any charges on that card for a whole year, and pay off that 2% service charge within the grace period. It might also have required that one-twelfth of that interest-free loan be repaid each month, but  I had stopped reading the offer at this point and filed it in the round circular file. In short, while @JoeTaxpayer's tale of how ""As a pay-in-full user, I've used the zero rate to throw $20K at the 5.25% mortgage"" is undoubtedly how things worked once, it is not at all clear that they still work that way. At least, they don't work that way for me.  Heck, once upon a time, for a period of about 3 months, you could earn 1.5% interest per month from the credit card company by overpaying your credit card bill considerably. Their computers then just ""added on"" 1.5% interest by multiplying your credit balance -$X by 1.015 and so you got 1.5% per month interest from the credit card company. The credit card agreements (and the software!) got changed in a hurry, and nowdays all credit-card agreements state in the fine print that if you overpay your bill, you don't earn any interest on the overpayment." Get a lawyer to put this in contract form, with everything spelled out explicitly. What is fair is what the two of you agree upon. My own suggestion: Divide the property into things which are yours, his, and shared, then have each of you be responsible for all your costs plus half the shared costs, but get all the benefits of your half. That would mean that if he rents out his half, all the rental income is his; if you decide to live in your half, all the savings of not paying rent are yours.  Each of you pays your half of mortgage, insurance, and other shared costs. Repairs to shared infrastructure should be done by someone both of you trust. If you agree the work is needed and he does it rather than your hiring someone, you owe him the appropriate percentage of the costs; the two of you will need to agree on whether you owe him for that percentage of his time as well. Make sure you agree on some mechanism for one person offering to buy the other out, or to sell their half to the other party... or potentially to someone else entirely. (Personally, I would try to do that at soonest opportunity, to avoid some of the ways this can go wrong -- see past comments about the hazards of guaranteeing a loan; this works or doesn't work similarly.) Does that address your question? As per my comments, I think this is up to you and how much work you want to put forth.  I do not feel it is trivial to provide documentation even with 90% of it will be the same among lenders. See this question: First answer, third and fourth paragraphs. You need to go as far as understanding the total cost of the loan, you probably need a good faith estimate.  I would also compare a minimum of three lenders. I'm not sure that you're considering all the options.   So you may not subtract $X from B, but you do compare NPV(B) to $Y.   Also, remember that we're not trying to figure out the return on B.  We're trying to figure out what to do next.  In terms of planning, the sunk cost is irrelevant.  But in terms of calculating return, A was a turkey.  And to calculate the return, we would include $X in our costs for B.  And for the second option, we'd subtract $X from $Y (may be negative).   Sunk costs are irrelevant to planning, but they are very relevant to retrospective analysis.  Please don't confuse the two.  When looking back, part of the cost for B will be that $X.  But in the middle, after paying $X and before starting B, the $X is gone.  You only have the building and have to make your decision based on the options you have at that moment.   You will sometimes hear $Y called the opportunity cost of B.  You could sell out for $Y or you could do B.  You should only do B if it is worth more than $Y.   The sunk cost fallacy would be comparing B to $X.  Assuming $Y is less than $X, this would make you not do B when it is your best path forward from that moment.  I.e. $Y < NPV(B) < $X means that you should do the project.  You will lose money (apparently that's a foregone conclusion), but you will lose less money than if you just sold out.   You should also do B if $Y < $X < NPV(B) or $X < $Y < NPV(B).  In general, you should do B any time $Y < NPV(B).  The only time you should not do B is if NPV(B) < $Y.  If they are exactly equal, then it doesn't matter financially whether you do B or not.