For a 12x20-foot (240-square-foot) deck, you can expect to pay between $7,200 and $14,400. This cost is based on a general price of $30 to $60 per square foot. The final price will depend on factors such as the decking material, design complexity, deck type, location, and local labor costs. Keep in mind that add-ons will increase the total; for example, adding an under-deck drainage system to a deck of this size could cost an additional $7,700.

Yes, but ensuring the deck has a solid foundation with adequate footings is critical before building, as the weight of a sunroom will put significant pressure on the structure below. Even if your deck is properly constructed, it may not be able to handle the extra weight. While some basic, lightweight, prefabricated sunrooms can sit on a deck with a strong existing foundation without additional supports, in many cases, the foundation and joists will need reinforcement. It is always best to have a structural engineer perform an inspection to evaluate the deck's quality and structural integrity. A professional can help you decide if reinforcement is sufficient or if it's better to tear down the deck and start over.

Footings for ground-level decks need to be at least 12 inches deep, and some areas require them to be even deeper. A standard range for deck footings is anywhere from 12 to 48 inches. The minimum footing depth is determined by an area’s frost line and soil conditions. If you’re unsure how deep to install deck footings, check local building codes to find the minimum footing depth in your area. 

Materials for a second-story deck must be weather- and rot-resistant, though the specific choices will largely be determined by your budget and style preference. Pressure-treated wood is on the lower end of the price scale, but it can be customized and stained the way you like it. Composite, on the other hand, is pricier for installation and has fewer options for customization, but you’ll save money and time in the long run because it requires little maintenance.