How much is that extra 5k worth when it takes 3, 6 months to get it rented and then you are over budget.

3-year PPPs are usually reasonably priced while also shaving off those extra 2 years...if you drop below 3, rate/cost can get a little gross.

Is it advantageous, or is it difficult and anti-foreigner with extra stamp duty and such?

(Give that to sellers, they picket $100k after paying off the loan.) and then pay them 4.5% interest (what they could get from the bank.) now you’re talking $750/yr extra interest, and it’s being paid to family. 

The only real offer I got through FF was for $1700 a month, and I'd rather leave it empty than take that and pay for the heating bills and the extra wear and tear of long term renters.

There was also extra bracing to keep the roof from moving further apart.  

That is assuming your DTI can cover the increased mortgage payment.If you have use for that extra $100k that you would get back right now then don’t wait.

Maybe they want bigger lot, more land, extra lots. 

So, the landlord makes an extra $1000 per month with the same fixed costs.  

Do an assessment of value now and after you get the ADU permitted * We have been in situations where $8k for permits could mean an extra $100k in value or $2k per month in rents.