need to borrow a bit of money for home improvements - its better to add to mortgage than take out a personal loan, right?

[fairylights]

as moving house would be a bad move right now we are partitioning a room to make an extra bedroom, and also getting a new boiler out of necessity. Need to borrow about £4000 and we assumed we would put it on mortgage but when we called mortgage company today they said this was ok but there was a set up fee of £350.. which seemed a lot for a relatively small amount.
we can see that personal loan APR's are way higher than the mortgage rate but can anyone tell me the pros and cons of either?
Sorry if its a silly question, we are sensible with money but not very financially literate
thanks!

bump

For an amount like that you'd be better with a personal loan IMO - easier to set up, no set-up fee or valuation survey, pick your own repayment period, no change in interest rate (which would be a pain if it went down but at least you know what your payments will be every month) & it's not secured on the house.

Our mortgage was paid off this year (faint hooray after 25 years!) & we need to get the roof done & a few other things - I am planning to get a personal loan to do that.