I don't want to seem older than I really am, but it doesn't seem to me like something actually "really-really" useful and I doubt that people would pay money for it.

The traditional way it was done (at least here in Italy):

1) due diligence (with your bank and a few others for comparison) to understand the costs of a mortgage, its duration and the monthly or bi-yearly repayment, this results in a sheet of paper with lines telling you in (say) 20,000 Euro's steps what the rate will be.

2) due diligence with your accountant about the tax credits that a given repayment may allow, as well in case of renovation you may (or may not) be eligible for a 50% or 65% tax rebate in 10 years

3) due diligence (with friends that did the same lately, sometimes with professionals) about the approximate range of costs for renovation (this usually results in anything between 600 and 1200 Euro/sqm depending on the amount of renovation needed)

4) due diligence with a notary about the costs of transferring property (taxes+fees)

5) due diligence with the estate agent to determine their fees (typically 2-3% of the amount paid).

At this point you sum:

1) amount you have ready in cash for downpayment (let's say 100,000, 75% max of total cash you have, or you need to have 135,000 cash to count 100,000)

2)amount that the bank is willing to lend you (and that you expect to be able to repay in 15-20-25 years, let's say another 100,000)

3) you have now an hypothetical budget of 200,000 (this is the money you can afford to give to the seller), to which you add notary and taxes (between 8,000 and 20,000) the estate agents fees (6,000) and some 5,000 more for moving and some other expenses connected to the moving, and you are going to spend roughly 10%-15% more, between 220,000 and 230,000, if the house is "perfect" and needs NO renovations, otherwise you detract the assumed costs for renovation, using the higher cost, 1,200 Euro/sqm, if on the local market the houses cost is 4,000 Euro/sqm, the size of the house you can afford is 50 square meters, amd you are within your budget of 200,000, if the house needs renovation you need to pay it no more than 140,000 to stay within budget.

Then you take a sheet of paper and on it you write:

the name of the agent and its contacts

the asking price

the status of the house (need for renovation, changes, etc.)

the address

the date of visit

your first impressions on the house

optionally you pin to this sheet of paper a few photos you took on your visit

These latter can be done (as it is usually done since - say - 30 years) on a simple spreadsheet.

A the end of your search for a new house you will have how many? 5, 10 or maybe 20 of such sheets (or spreadsheets), anyway a small number that you can (print and) spread on a table to view/review/compare to make a choice.

Your app will be (at least for Italy, or however any country where the tax code and procedures for buying a house are complex):

1) incorrect in the calculation of the mortgage (unless you have access to many banks conditions/contracts) this could be 5% but up to 10% error

2) incorrect in the amount of taxes and fees to be paid (unless you implement also tax software) there could easily be a 100% or more error in this

3) incorrect in the amount due to the estate agent (unless you can connect to the estate agent fees structure)

4) incorrect in the costs for renovations (unless you have access to valid prioces/costs in the specific area) this could be a 50% error, besides the possible access to tax rebates

5) most probably correct in commute times (which will be the same ones you can get from google maps or similar)

I can see no particular advantages (that I would pay money for) but - worse than that - I personally would not trust your app to make accurate calculations, and when you buy a house it is a lot of money, often the whole amount of savings of a family and a substantial part of their future spending for 15-20-25 years, even a 10% error on the total could be affecting the future of the family.

I mean, I like the n "price comparison" sites for household items, but those are 10-100-1000 Euro, even if there are mistakes and you come out choosing an item that costs 1200 Euro instead of 1000, it doesn't make a change, choosing a house that will cost in total 240,000 Euro instead of 200,000 may instead be a big issue.