Can i alter my internal fire doors?

[BeQuirkySquid]

I own a flat on the 3rd floor of an apartment block, which has 5 floors in total (900 year leasehold and own a share of freehold with other owners). We are getting the place recarpeted and underfloor heating installed. This means that the four internal doors will need to either be shaved at the bottom or the entire door frame and doors moved up by 30mm to accomodate the new height.
The internal doors appear to be fire doors (self-closing chain, intumescent strips) but don't have any branding or stickers, so difficult to tell if it is a proper fire door or how much they can be shaved. The building was constructed in 1999 so unable to get more info if these are regulation fire doors or why they were installed at that time. Have spoken to local council and Fire authorities and no-one is able to provide consensus on what to do.

I am considering a few options and would appreciate if anyone has experience or advice on what my options are while sticking within the fire regulations or also if I am misinterpreting the rules:

1. We can just shave off the 3cm from the bottom of the door and stick on some paint or something to make it fireproof? (cheapest option)
2. Move the entire frame and door up by 3cm. Super expensive and not sure if this will affect the fire safety rating
3. Install new fire doors. I don't think I can afford this.

Doors like those are not that expensive.

There is a limit normally as to how much you can shave off the bottom of a fire door and it will be in the manual.

Check the top of the door for all of them and see whether there is a sticker on top that details the door type. Some of mine have them on and some don't.

In the terms of your lease and under fire risk assessments, you'll probably find that you will have to have them as fire doors.

I have doors I can only have as fire doors in my flat. With respect, if you can afford to re-carpet and install underfloor heating, you should find the extra funds for fire doors - your future self or those in other flats could one day be thankful for this.

Yes, you can change them but only for other fire doors. We are in the same position as you and we are changing our internal doors in August as the old doors that the sellers put in aren't fire doors.

I'd do 1. We did this in our old house. All fine.

Thanks @Clearinguptheclutter. Which paint did you use?

Non fire doors are 35mm thick whereas Fire doors (FD30) are 44mm thick.