You can add plywood underneath the cushions to support them.
What part of your couch is sagging? Here are multiple problems & solutions for specific parts of the couch that are sagging, see if one or more applies to your situation:
There is good information here:
You may be able to just simply create a center support beneath the couch. I'm guessing if you flip it over to look underneath it the center supports have cracked or are sagging. I would use 2x4s to the floor in the center to give you the support.
Check to see if a spring is broken, replace it using directions on YouTube. https://www.youtube.com/results?search_query=how+to+fix+a+sofa+spring
They also make a folding composite piece that you put under the cushions if you can't do that. https://www.carolwright.com/seat-saver/38653.cfm?clicksource=search
My sofa was too low when I messed up my knee, so I put a slab of 3" firm foam under the cushions that really made it more comfortable and raised it up just enough for me to get up an down easily. I got the foam at the local home store, and cut it to fit using a bread knife. Being gray its kinda invisible under the cushions.
here is info https://www.youtube.com/watch?v=KOO8Oy_v9Ls
Cut a piece of plywood big enough to fit under the cushions and then place them back on top.
How to Reinforce a Sofa's Bottom
Hi Janny: I had the same problem. I turned my couch over and saw that the springs were loose. I tightened them and then reinforced them with rope. Here is a site that I used for my instructions:
Here are a couple more sites that may help:
This should help: https://www.youtube.com/watch?v=0KlyEhMK-Jg
Hi Janny. Cut a piece of plywood to fit under the cushions of your couch. Put it in place and put the cushions back on top of it. Good luck dear.
Your stretch fabric broke it needs to be repaired you could put a plywood in it but it will be hard
Hello. We have been there with this problem ourselves.We’ve rehabilitated our Natuzzi by flipping the couch over on its side removing the bottom thin black fabric (the dust cover) and addressing the interior structure. We ended up putting a piece of wood ( screwed in ) to support the inside the frame to help correct the seat from sagging, or if needed extra foam filling could be applied if needed. Reattach ( staple gun) black dust cover and enjoy the revitalized cushion with proper support.
Not sure if this is a viable solution for you, hope it helps.
Sorry not too many options for you. Is the frame of the couch broken or just the cushions worn??
I found this! https://www.pinterest.com/search/pins/?q=How%20to%20fix%20sagging%20bottom%20of%20couch%3F&rs=typed&term_meta=How%7Ctyped&term_meta=to%7Ctyped&term_meta=fix%7Ctyped&term_meta=sagging%7Ctyped&term_meta=bottom%7Ctyped&term_meta=of%7Ctyped&term_meta=couch%3F%7Ctyped