What’s happening to my walls?

It sounds like the walls were done with the dot and dab system , the plaster skim coat has dried out but the dabs of adhesive have not, they will dry out eventually, but may need a repaint. If the work was done during damp weather it would have been advisable to have dehumidifier in place for at least a week .

Ive come across this many times , i would put money on it that its happening on a outside wall , what you are seeing is the dab that the plasterers have used to keep the blasterboard on the wall , , it will dry out , need a good hot summer then stain block the marks and repaint

This could very well be due to damp trapped in the room whiles having your extension done. It would of been recommended by our team especially to have a dehumidifier in there for a week to suck any of the damp out of the blocks bricks and plaster board. You could always put 1 in the extension now to help reduce damp dot and dab stains coming through but a hot summer with the windows open will suck the dot and dab back off the boards and you will be able to mist coat it then re paint. We hope this helps. Regards Team CJL

Definitely the plasterboard adhesive, last to dry. It will dry out then stain block and re-paint