A modular building system comprising a plurality of building elements, which are assembled to form a building section, such as a wall, a floor and/or a ceiling, said building elements, comprising an elongated insulation panel having a first side and an opposite second side with two oppositely positioned end sides (25, 26) and two oppositely positioned panel contact sides (21, 22) therebetween, at least one of said elements comprises: cover profiles, each of which are adapted to cover the width of the first side and the second side, respectively, wherein at least one of said cover profiles has a base portion being attached to the first or second side (23, 24) of the insulation panel, and has first and second side flange portions on each side edge of the base portion; said first side flange portion having a tongue and said second side flange portion having a groove complementary to the tongue adapted to receiving and interlocking the tongue of a neighbouring building element and thereby assemble the two neighbouring building elements (1, 1') said modular system being characterized in that the tongue of one side fits into the groove of the other side of the abutting cover profile so that the interconnected flange portions in cross section define a dove tail wherein each of the flange portions comprises an inclined inner flange portion extending from the base portion to an intermediate portion parallel to the base portion, and from which an outermost side flange is provided which is perpendicular to the base portion, and that in said outermost side flange the tongue or groove, respectively, is formed. said building elements, comprising an elongated insulation panel having a first side and an opposite second side with two oppositely positioned end sides (25, 26) and two oppositely positioned panel contact sides (21, 22) therebetween, at least one of said elements comprises: cover profiles, each of which are adapted to cover the width of the first side and the second side, respectively, wherein at least one of said cover profiles has a base portion being attached to the first or second side (23, 24) of the insulation panel, and has first and second side flange portions on each side edge of the base portion; cover profiles, each of which are adapted to cover the width of the first side and the second side, respectively, wherein at least one of said cover profiles has a base portion being attached to the first or second side (23, 24) of the insulation panel, and has first and second side flange portions on each side edge of the base portion; said first side flange portion having a tongue and said second side flange portion having a groove complementary to the tongue adapted to receiving and interlocking the tongue of a neighbouring building element and thereby assemble the two neighbouring building elements (1, 1') said modular system being characterized in that the tongue of one side fits into the groove of the other side of the abutting cover profile so that the interconnected flange portions in cross section define a dove tail wherein each of the flange portions comprises an inclined inner flange portion extending from the base portion to an intermediate portion parallel to the base portion, and from which an outermost side flange is provided which is perpendicular to the base portion, and that in said outermost side flange the tongue or groove, respectively, is formed.