my extension wall is 12" thick with no cavity and have rising damp, outside is dashed and wanted to know if it can be treated just from inside or will i have to hack off the dashing, thanks.

A wall that thick needs to be done both sides, the dashing shouldnt breach the dpc level. A bell cast bead should be fitted just above dpc level, and check that the external ground level is minimum 150mm below finish floor level.

Yes you can treat from inside without hacking off the exterior dashing.(as long as there is no visible or obvious damage or ware to the exterior surface). You would need to remove interior plaster and render from floor/wall junction to specified height(Min height advised 1.2-1.5m), drill and inject into interior mortar joints, then re render and plaster, this will stop your rising damp from inside without damaging outside, hope this helps.

Assuming the water is using the pebbledashing to capilarate up over your damp course, the best action would be to remove the dash from below the damp course. Treating from inside would be a short term bodge. Also consider that a wall without a cavity is called "solid wall" these tend to be very cold and warm air hitting a cold wall can easily cause mildew. But you would expect to see this around the windows initially. Good luck

YOURE going to hack off the dashing and you are asking this question ,why not get a professional round to do it