Buy-to-Let Major Repairs and Improvements

Like for like replacement of worn out building components is an allowable expense against rental income for tax purposes, but improvements are not.
In practice it can be a little hard to distinguish them, as almost any replacement would be with a modern equivalent. Your accountant may be able to advise.

Costs that are allowable against rent cannot also be set off against capital gains tax when the property is sold.
There are no published sources of data on the amounts spent on major repairs and improvements in the private rental sector.
In social housing the average spend works out at around £225 per unit per annum.

Most buy-to-let landlords spend somewhat more than that, usually between tenancies.

They also have to include the replacement of white goods.

Their costs are more likely to average £1,000 or more per annum, and to be spread very unevenly.