financing a second house with mortgage from my first.

Yes. You could either increase the loan while staying with your existing lender (who will typically allow the loan to increase up to

But be warned. This is not the best time to be jumping on the BTL bandwagon. D'ya wanna buy one of mine? :-)

Do you really think it likely that he will find a better deal on a straight BTL than on a residential re-mortgage? Granted if he went for a substantial further advance from the existing lender at SVR that might not be very good, but there remains the option of going for a good discount deal in residential mode, which is likely to beat any discount deal in BTL mode.

Umm no, and I haven't said any such thing. Just told him to shop around for a residential re-mortgage.

So you did, but it came out in a slightly awkward way, almost as if you were saying "It's a bad idea, but if you want to do it anyway, then make sure you shop around". The way the original query was phrased didn't exactly help either, because the bit "(with the original mortgage)" can be taken, or not, to mean he was thinking only of taking a further advance from his existing lender, presumably at SVR. Clearly if that's what was meant, that would indeed be unlikely to be optimal.

Unfortunately the ambiguity in the meaning of "remortgage" is to blame here. Generally in the trade nowadays it means "borrow elsewhere instead", not just "borrow more", though often in practice it involves doing both.

What worried me was that if his parenthesised bit did not mean staying with the existing lender, but was just a clause more particularly describing the first property, then your first sentence could be taken to pour cold water on the whole idea of remortgaging the first to finance the second, not just on the specific idea of doing it with the existing lender.

Likewise, the first sentence of your second paragraph is also open to misinterpretation, depending on what the reader understands by "new borrowers" vs "existing borrowers". If they think an existing borrower is one who has a mortgage loan, and a new borrower is basically an FTB, they may gain the impression that not being an FTB will close the doors to the best deals (which it sometimes in fact does), and if you can't get as good a deal by remortgaging as you can in a new-buy, they might think that's true also of a new-buy BTL.

Just trying to clear up any possible confusion.

Pedantically, Yours etc,