Something

new gPickup[MAX_PLAYERS+MAX_HOUSE+30];

#define HOUSE_LIMIT 2000
#define HOUSE_START 1000

gPickup[pickup] = id+MAX_HOUSE+HOUSE_START; //set value when house is created
//below 1000 is a house

public OnPlayerPickupPickup(playerid, pickupid)
{
   if(gPickup[pickupid] >= MAX_HOUSE+HOUSE_START && gPickup[pickupid] < HOUSE_LIMIT) //check if pickup has a house value and below 2000
   {
      new houseid = gPickup[pickupid] - (MAX_HOUSE + HOUSE_START);
      printf("Entered house id: %d", houseid);
   }
   return 1;
}