The average cost of a sunroom ranges from $100 to $350 per square foot. Building a 3-season room costs between $8,000 and $50,000 while adding a 4-season room can range from $20,000 to $80,000. The cost is usually between $5,000 and $30,000 for a sun porch or prefab sunroom addition. If you’re considering a glass solarium, the price can range from $30,000 to $150,000.