How wide does chimney breast have to be to fit range cooker?

You will need at least 4ich brick each side. To surport the lintel . If you not got that. Then a block wall on one side with a return . To suport and finish to ceiling. Mark innes. Innes construction

As above you also must check manufacturers instruction book to find out what clearance is required around the appliance and also building regs and go with instructions if the gap is greater. work with a registered installer who will give you the advice you need remember. Do not expect him to call in and give you the advice for free, you will save on labour cost making the hole and msking it look pretty. The installer has to install it and sighn it off or you will get into trouble of something goes wrong .insurance will not pay out or worse if a death is involved.

Under doc j building regs you need 200mm either side tbh .If this can't be done as other guy said block up either side so you can easy job . Mark Belton stoves and chimneys ltd

In my opinion you can’t go off document j, That’s just a guide the stove you pick will tell you the sizes needed. Make sure you have a minimum of 100mm bearing on each side of the lintol you could fit it your self if you have some knowledge and pay around £150 to get the council to sign off. Could cost more depending on we’re you located,.