Quite a lot of the information in this thread isn’t really right. I am a professional landlord, and there’s a lot to be considered.
Firstly, you may not be able to get a buy to let mortgage on that property. Mortgage companies will want to see much more rental income in excess of the payment. You may be able to get consent to let on your current mortgage. I expect my mortgage to be 25% of rental income, although my portfolio can run at 50%
Having said that, a buy to let mortgage will be interest only, which could be MUCH lower, as you generally only pay the interest, not a repayments. You are expected to make separate arrangements to pay off the actual loan (many people just remortgage forever until they sell. Some make other investments or have multiple buy to lets). You may struggle to get a buy to let mortgage depending on your income, or if you do not own a home otherwise or on a flat. They are very peculiar rules.
You can no longer declare your mortgage interest as an ‘allowable tax expense’ and then just pay tax on the profits after you’ve taken it off. You calculate your profits and then apply 20% of the mortgage interest as tax relief. This makes precisely no difference to the amount of tax you pay, unless you are a higher rate tax payer.
The rules and regs to stop landlords letting complete slums have (quite rightly) become very stringent over the last few years. You will need annual gas and electric checks (electric might be two years) and lots of things like fire extinguishers and your EPC must be at least band D (I’ve a feeling its going up to C, soon too). You will need to hold the deposit in a deposit protection scheme. If you have an agent, they will take care of this. And they will charge you 14% to do that.
You will rarely get the full amount each month. There will be checks, certificates, agency fees, advertising costs, insurance, tenancy agreements fees repairs and maintenance, and whatever else the agent has dreamed up. There are also pretty strict rules on what you can and can’t do if people don’t pay. Some months, people won’t pay. It’s inevitable. Sometimes, it will not be let at all. A local agent will be able to give you a better idea of some these costs locally to you. My insurance will be different to yours, as the property is different and I don’t have loss of rent cover. Your costs may be vastly different in if you are in London, say. It’s not helpful to give you mine!)
There is a certain amount of work to be done. You probably don’t need an accountant, if you can manage to fill in a tax return, and it probably won’t be loads (until some crisis or other hits. Sooner or later, one will!) Refinancing is a right pain in the proverbial and maintenance etc is ongoing.
I don’t think its a bad thing to do, but it’s not remotely the passive income people imagine. I still like property as an investment, because as well as income, you get the intrinsic rise in value which is often profitable and generally beats inflation. However, property is a very fixed asset. You cannot sell off a bit if you need extras funds one year, or diversify part of it if its doing badly, or take advantage of a surge in growth. You have to dispose of it all in one go. Sometimes that’s easy, sometimes it takes years, sometimes you make a loss, depending on the market and whether you can wait. And you always have to pay capital gains tax. (With say, shares you can dispose of a portion of your portfolio each year to keep you under the CGT tax threshold). It’s best to think of it of at least a side hustle, if not a part time business - do you want that? Sometimes it will be a case of just cashing the cheques. But sometimes it will be a pain in the proverbial. And there’s no telling when, either!
With the figures you have given, it’s not remotely viable. However, the figures aren’t really right, as the finance you need isn’t what you have. However, for all I know, you are happy to take a income-loss on it, and just let it accrue value while you pay it off (as you won’t have housing costs yourself). That’s a completely valid investment to make.
Also, consider what is going to happen to DMs house. If it’s your home too, how will that be reflected in the ownership? Are you going to part own it, is she going to leave it to you in her will? Can you pay inheritance tax from additional funds if she dies suddenly? What if she develops mental health problems and randomly leaves everything to the cat (or your siblings. Do you have siblings? Will they resent you getting the house, or a larger share of the house, as you will be helping out DM or will they still expect it to be equal? If you do get a larger share and they are not well off, its likely they will resent the additional wealth you could definitely gain here) or needs years of nursing care and the house must be sold to pay for them?
In your situation, I would think very carefully about whether I wanted to live my mother (In my case - Hell, no!) and if I did, I would insist that my mother put the house in our joint names as tenants in common so that the house automatically passed to me when she dies (it never forms part of her estate in this instance) and so cannot be willed elsewhere OR gifted it to me now, and a life interest is held for her (so it cannot be sold or she cannot be made to leave unless its unsuitable and comparable accommodations bought with the proceeds. I think that is possible with a living person) then it could not be willed away in fluctuating capacity or (sorry, it happens!) malice, forcibly sold to fund care costs and you won’t have to pay inheritance tax if she lives for seven years after gifting it to you (whether or not this seems likely would most influence which option)
I would then get a get a better mortgage (At a very rough guess I’d say you could halve it, but it depends on LOTS of info I don’t have here so that’s a real stab in the dark) get the flat managed by an agent and suck up the costs and more or less forget about it. I would keep the income from it separate, and drip feed it into an ISA (where I would probably take the tax savings each year and give myself a treat) and let it grow. When it comes to remortaging I would look each time (usually 2 or 5 years. I wouldn’t fix for longer, it’s pretty uncertain what will happen the moment) at the interest rates and decide whether to pay down the debt with no penalties (you just remortgage for less) or to reinvest the ISA money into more property or other investments.
I would also take the money you are currently paying on my mortgage and divide it into thirds. I would put one third into my pension, drip feed one third into cash savings or higher risk investments and and one third into treating myself to things I’d always wanted. Your living costs will be similar (in many cases less) and you should be able to sustain both an uplift in lifestyle and good financial planning. BUT it comes with risk attached, so you need a safely net (for example, if you lose your job, you will not be entitled to benefits of any kind. Also, there was no help for landlords during coronavirus. None. If you relied on property for income and it didn’t happen to be let, or the tenant couldn’t pay, tough, you’re on your own. So you’d better make your own safely net, lady!)
All this is assuming you are single, sub fifty and don’t already have a big pension or lots of investments or are planning/have a family. In which case, I imagine you would be asking different questions. Oh, and I wouldn’t get married. (What would you do if you did meet someone - continue to live with DM? Marriage would complicate things financially. I would keep any finances very separate. I’m aware there are IHT benefits, but I don’t think they outweigh the risks of need to share assets in the case of relationship breakdown, and I think there are better ways of avoiding them). I’m not having any more children, and no chap has played any part in supporting my family, so I don’t feel any financial obligation to support a future partner. I’d assume they’d be able to continue to support themselves if I they left/I died, although if I knew otherwise I might make some provision, but I would still expect to leave my daughter considerably better off than any partner)
This could leave you in a very comfortable position, when you retire and when your DM dies (hopefully peacefully in bed, many years hence etc etc etc) and living pretty comfortably up until then, too, if you are happy to take the lifestyle of sharing a house and somewhat prudent with your investments. There is risk here though, and you should be aware of it, comfortable with it and have some sort of plan in place for it.
Good luck!