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No, if you are looking at getting a mortgage on it, you can get it on residential rates, if you are to consider it as a secondary residence. They'll assess your affordability for running both mortgages and running costs. Assuming you can afford it, you can have unlimited residences (though lender dependent)
If you aren't going to rent it out, you shouldn't be allowed a BTL mortgage either, as the BTL mortgage is driven by the rental that they expect you to receive.
Simple BTL mortgages aren't actually much more expensive, but for these you would typically require a larger deposit, 25% or more, along with being limited by how much your rental income would allow you to raise.
In areas of London, due to the lower rental yield vs value, in some cases, deposits need to be 40-50%.When you get to limited company/multi unit blocks/HMOs/Holiday lets/lower rental calculation BTL mortgages, they start getting more expensive however.
Also you would need to factor in 3% second home stamp duty, whether or not you buy it has a second home or a BTL.
If you buy a second house and don't rent the first out (say you just use it as an office or something) do you still get stung with more expensive BTL mortgages because it's not considered your primary residence? Or are mortgages more expensive only when you are actually letting the place out?