Impossible to guess without a ton more info. Post up some pictures for starters.

What are your goals for the project? Do you want more living space? Love the outdoors, but hate mosquitoes? What part of the country are you in, and which way is north?

Do you currently use the deck? Would you also need to add a deck off the sun room? Do you ever use a grill?

How would the sunroom roof tie into the existing roof?

"Decks are not built to interior habitable space standards, so it can not be done like you are thinking."

Look again W.T., please:

Regardless of live loads a deck is designed for - your footings most certainly are not designed for the weight of the roof, not to mention the rest - insulation, windows, appropriate flooring, walls. There are also different inspection requirements for decks vs. enclosed living space, so I doubt any jurisdiction would say an old deck would be OK to reuse for this purpose. The angles of your deck would also make it a challenge for a roof.

You can put a sunroom there but it will require a tear down and rebuild.

The amount of light the roof would block is dependent on which way you face and if you have any trees. Given the amount of windows you have now I wouldn't think the reduction of light would be a big deal.

Regarding cost. We just priced out a screened in deck about a year ago. In my area, it was going to cost close to $100,000, with the cheapest trex. We opted for the deck instead, which still cost $40,000. I don't know how a screen porch would compare material wise to interior finishes, but I can't imagine it would be less.

Our foundation is helical piles, and they were installed based on loads required to the appropriate depth. We cannot add significant load to our deck without redoing the foundations (rebuilding).

anj and all are correct. The footings and all that were required to build the porch were amazing to me. The builders said our porch has a stronger foundation now than our house which was built in 1960 . Very interesting to me...and a tad disconcerting. ;-)

@cyn427 (z. 7, N. VA) thanks so much for sharing! wow your new porch looks amazing! and great tips about the skylights and double doors, I will definitely keep them in mind. Do you live in a state where it's not too cold? Our state actually has a pretty long winter. I was initially leaning toward a screened porch (as opposed to total enclosed sunscreen) but I'd also like to use it during winter.

@anj_p ahhh this seems to be a much bigger project than I had anticipated, if we need to do a total tear down and rebuild. Your estimate of 100,000 for a screened in deck, does that include any glass or all mesh windows?

@AnnKH our living room does not have a large window - there are several narrow corner windows but I would like an unobstructed view of the outside. I was hoping an enclosed deck/sunroom can provide me with more views of our yard.

We live in New England so the winter here can be long, that's why I was hoping to get a 4 season room isntead of just a screened in deck. We do use the deck when the weather is nice, so possibly we will build a patio as an extension of the sunscreen.

Definitely not inexpensive, but if you're planning to be in your house a long time and would really enjoy the space it might be worth it. IMO it would help you sell your house faster as it would be an appealing feature, but I'm not sure how much of the cost would actually come back to you. Someone would buy your house if it didn't have this feature. Ask a local realtor for their opinion if this is a concern of yours.

No, the 100k estimate was not including windows. The cost is why we decided to do a deck instead. We live in MN so have similar weather. In a few years we may consider adding drainage under the deck and screening underneath, which is a less expensive project. But we didn't really need additional indoor living, and just wanted some nice outdoor living.

Our neighbors have a sun room on their house. It never gets used. I would strongly consider whether you need additional living space before you start this project, as that is what you are adding - more living space. It would not be nicer outdoor space. I doubt you would get anywhere close to your money back on this project, so you need to do it for your own enjoyment (you'd probably get approximate cost/sf for homes in your area and level of finishing * the area of your sunroom back, as you'd be adding livable space to your house).

You could also convert your breakfast nook to a sun room of sorts and just use your dining room fo rmeals. Put a couple club chairs and a nice side table, and you're done. Then you can still have nice outdoor living space on your deck.

Imma, we are definitely not as cold as you are up there. I do love New England and would move north in a minute if winters weren't so long. I grew up in Pennsylvania and that was lovely and more snow than we get here. I love snow. We are in Northern Virginia outside DC. Although we get cold at times (went down into single digits a month or so ago), we are mostly high 20s F to low 40s F in the winter. I think we will get a good 7 months when we will be able to use the porch-fingers crossed. I plan to spend lots of time out there! Thanks for your kind words.