I have recently moved into an old cottage with a 50 year old extension, on the corner of two of the walls one by the patio doors and one by the dining room door there are brown stains, we have had three surveyors ref possible damp and whilst they all said something different none pick these up as damp. We have three plasterers for quotes and one says do nothing one says you need a lime plaster and the other suggests an injection damp course. I'm really confused can anyone help. I have attached some photos.