How difficult is it to build a side extension and a pitched, tiled roof with no side scaffold access

Assuming it is fair face work brick cavity wall, I would estimate for the wall that can't be accessed it would take double the time and therefore double the cost to build. If it was an externally rendered blockwork wall I would guess a little longer and a little more time. The roofer's job should not be as affected as much as long as long as he has good access from the rear of the extension.

Building the wall overhand ( not unusual ) would incur an additional 100% labour charge and you would also need to use a full fill cavity insulation as celotex bats would not be suitable for this application constructing the roof is predominately done from an internal scaffolding so not to much of a problem, however when it comes to the soffit facias & any gutters that maybe on the side with limited access this could be more complicated especially if no access at all like wise the roof tiling would be that much more awkward and safety features to enable the tiling may come at a slight premium personally if i were pricing such a job i would be looking to increase my rate by 1/4 to 1/3 subject to exact detail and expect the extra time taken to relate to this Regards J Hartwell

I'm guessing by the sizes given in the question it is a single story extension? If so yes it can be done, the brickwork and block work can be built overhand. Any good bricklayer can work both ways. Doing the roof will be a bit awkward but it's not as bad as you would think, again a good builder would get round it . You will be saving on the cost of not needing the scaffolding on the outside. Maybe if you budget a extra £350 on the job it will be more than enough

I wouldn’t expect the work to take approx 15/per cent longer on outer face brick wall as overhand work ,, so a small premium would be added ,in the original costing the amount of scaffold would have been catered for , the inner leaf block work wouldn’t be an issue ,cutting of the rake ( angle ) of the roof wouldn’t be an issue , the roof construction can be done adequately from the wall plate scaffolding however provision must be made so no one could fall sideways off , suggest a horizontal two bar restraint , which is quite common , where possible avoid facia board as in future unless it was upvc there would be no access to maintain , I would spec bricks cut to pitch , then the undercloak , then batton and tiled in normal manner with flush jointing on verge , if facia board was to be used it could be pre cut on ground and secured into noggins thru brickwork , still possible from an overhand position