He increases the length to 13+2=<<13+2=15>>15 feet
He increases the width to 18+2=<<18+2=20>>20 feet
So the rooms are 15*20=<<15*20=300>>300 square feet
So he has 1+3=<<1+3=4>>4 rooms of this size
So he has 4*300=<<4*300=1200>>1200 square feet of rooms this size
He also has one room of size 300*2=<<300*2=600>>600 square feet
So he has 1200+600=<<1200+600=1800>>1800 square feet of rooms
#### 1800