It's not number of houses, it's number of houses where people want to live.
"house prices would not be increasing in areas of population decline" -> the causation could be running backwards: a family of 2 (maybe 2 retired people with accumulated wealth) can outcompete with a young family of 4 for the same unit, that drives prices up and population down. I'm not saying this is the case, but decreasing population not being associated with decreasing housing prices does not mean that increasing housing prices is not caused by housing scarcity
"house prices would not be increasing in areas of population decline" -> the causation could be running backwards: a family of 2 (maybe 2 retired people with accumulated wealth) can outcompete with a young family of 4 for the same unit, that drives prices up and population down. I'm not saying this is the case, but decreasing population not being associated with decreasing housing prices does not mean that increasing housing prices is not caused by housing scarcity