Hi,
Welcome to Pet Talk!!!

My little brother has a Great Pyrnes New Foundland mix,5Yrs,that is as sweet as can be...He sounds a lot like your GReatP. but I cant undestand why yours suddenly became agressive,my GSD did this but just to other Male dogs(except the GreatP. which is kinda weird ,i guess the GSD looks up to him) Maybe its stress????Because you are adding onto your house and i do figure that other people are coming onto the property delivering items ,and you had said "perhaps he just missed us being downstairs with him all the time" this could have also put stress on him,not being able to be with you alot as much as he had.....These things could have put stress on him.....WEll hope it gets better and Welcome To PetTalk!!!

Kay


FCR