Here is the code that traces the house ground

Click to reveal..

Code:
if(mouse_left==1)
{
	a=1;//start at the first vertex of the house
	nevar_celt=0;//Can i build a house here
	while(a<=5)//check if can build
	{
		b=a+1;//secodn traced vertex
		if(a>=5){b=1;}//Trace the first node, so that a square is traced
		vec_for_vertex(node_pos,building_name,a);
		vec_for_vertex(node_pos2,building_name,b);
		if(c_trace(node_pos,node_pos2,IGNORE_ME|IGNORE_PASSABLE)>=1||c_trace(node_pos2,node_pos,IGNORE_ME|IGNORE_PASSABLE)>=1)
		{
			nevar_celt=1;//can't build the house here
			set(building_name,LIGHT);
			building_name.green=0;
			building_name.blue=0;
			building_name.red=255;
			wait(-.1);
			reset(building_name,LIGHT);
		}
		a+=1;//Continue to the next vertex
		wait(1);
	}
}