Im presuming this is internal.This archway is hiding what was once a normal door size opening.it has been created by removing the old frame and adding an arch former kit.if you were to remove this (it is only thin mesh fixed to the sides and top of the opening and covered in plaster) you will be left with an opening that will take a door frame.you will also see if there is a lintel in place (most of the time there is unless its a stud wall,in which case it is not required).the mesh can be removed by hammer and bolster,scutch comb or crowbar then once the frame has been fitted its just a case of making good the plaster around the frame.if in doubt get a builder to take a look but its an easy job and will not cost much at all.

No lintel is required
Use timber to frame the opening to the correct measurements for the door frame
Plaster board the stud work than add the door frame and hang your door
Regards
mbcarpentry

Hi,

If you are not sure what you're doing. It might be worth getting somebody to look at it first, just to be on the safe side.

Yes you would need to fit a lintel, but only a light weight one as it only needs to support the brick work you are adding. Unless you knock out the arch brick work and fill in with brick work courses. Then you would need a heavier duty lintel because the weight is being spread. The lintel has to sit in the brick work 150mm either side.