The reasonable would be to take the cost of maintaining the house (rent, heating, internet, council tax) and divide it by the number of people living there. If it is just your mother and you, it is two.

You can always look how much it would be to rent in your area, I would think in many areas it would be difficult to find something for 400 a month, but it really depends on what is normal locally.

It also depends how old you are an how much income you have. If it does turn out that you can live cheaper somewhere else, just move out.

It is reasonable when you consider that you ave no furniture to purchase, no electric, no CT, no central heating, no washing costs and I am sure you don't buy and cook all your own food and even if you do it is because you choose not to eat family meals....... go look for somewhere else to live and then you will realise the cost of living as £350 in rent alone is not the only cost of living

are you paying the rent which you claim is only 350

but now she wants 400 for food(that's what board is), tell her you will buy your own food and keep it separate

The rent is £350 - then add council tax, bills etc.

Yes, it's unreasonable to ask more than the entire rent - it would be reasonable to split the rent, bills and tax

Consider moving out. Then she will have no board.

It does not really matter whether this is reasonable or not. How much would it cost you to live on your own?

Can't you ask her to explain why she wants an increase?