Get a good home inspector. Make an acceptable report one of the conditions of your offer. Don't even consider an offer without this condition.

You will probably also need an estimate of how much it will cost to fx this problem. That will depend on the source and the extent of damage.

It may be minor and you can fix it yourself, but if it is structural, especially if the source is longstanding leak from overhead, the solution may cost more than you want to spend, unless you get a really good deal.

Have you done all your research on synthetic stucco? I've heard of many problems with synthetic stucco, it allows moisture to become trapped beneath the stucco without any egress if it's not done properly. This results in significant damage to the home which may not be totally visible. Get a good inspector, especially one that is familiar with what to watch for in synthetic stucco. the rotting wood you see visible may only be the beginning of your nightmare. Check it thoroughly before you buy this home. Good luck.

Coincidental timing... I'm taking a lunch break from tearing out some rotted wood near what used to be a door to the deck. They didn't flash the ledger board for the deck, so it trapped water against the siding, both of which rotted. It stayed wet enough to even attract some termites (no live ones, but tunnels and pellets, even in the redwood siding) and rot some of the rim joist. Our damage is mostly on the outside, but the same lessons are to be learned.

Water is the enemy when it comes to buildings. It's very persistant, spreads and wicks in ways you wouldn't think possible sometimes, and will silently damage wood, sheetrock, non-galv. steel, etc. And if it sticks around long enough, it will bring its friends, mold, mildew and insects.

I would guess that the patio door is at fault, either tthe flashing below it, the door itself not shedding water correctly, or if you're really unlucky, the flashing above it ("Mommy, why's it raining inside the door?"). It's important to find the source of the water and stop it before you do any other repairs, or the repairs will just get wet, too.

From the looks of your picture, it's been fairly wet for a long time. The subfloor planks need to be replaced obviously, but check very carefully in the joists below for rot. If there is just a little, you can scrape it out, make sure it's dry, and fill it with bondo. If it's a little worse, you may need to do that and then scab another board along side the joist to strengthen it. If it's really bad, well...

But be sure to fix the source of the water first.

Claudia,
Take a look at the info below.

Frankly from what I have read about synthetic stucco I would not touch a stucco house unless I was absolutely certain it was "real" old fashioned cement stucco.

Anne

Here is a link that might be useful: EIFS Legal Network

A new door, replaced flooring are not cheap. I have been through it. The door is not installed nor flashed proprerly. Do you want a home with one very obvious quality defect? What other not so obvious defects do you not know about? Be leery of under estimating the cost of repairs.