Golden Circle and South Coast on the same day?

I've not done it yet, but am planning for a few months out. We felt that was too much and decided to split it into two days. You could certainly make the Golden Circle in that amount of time, per our calculations, but I'm not sure about the South Coast. Our research shows that round trip it is 4.5 hours. So, with seeing the sights that seems rushed. But, again, I haven't done it yet, so cannot speak from experience, just from a lot of research on Google Maps, looking at guided tour routes/itineraries, and reading various travel guides. Definitely interested to hear how it goes!

You could certainly drive it but you wouldn't have time to stop and see much of anything. You are going at the height of the tourist season and there will be many cars, tour buses, and the regular caravan of trucks on the road, the parking lots at each sight will be quite full, and you'll have to do some walking to see the waterfalls and the black sand beach.
Do you mean drive from Reykjavik to the Golden Circle, the back to Reykjavik, then drive the south coast? Reykjavik more than an hour west of the Golden Circle so you will be doing A LOT of backtracking to go all the way east back to Vik. If you really want to attempt all this in a day you should go Reykjavik, Golden Circle, Vik, and then back to Reykjavik. Don't go all the way back to Reykjavik to rest...you'll be going west for an hour and then retracing your steps for an hour east again along the same road.
Driving distances in Iceland shouldn't be underestimated - distances are quite vast, even the main roads can be narrow, there are single lane bridges where you have to wait for oncoming traffic to clear before you can cross, and the wind and weather can be quite vicious if you are unlucky.
If you can you should see the Golden Circle one day, spend that night somewhere on the southern coast (Hella or Vik?), and then see the sights of the southern road on the second day.

"Do you mean drive from Reykjavik to the Golden Circle, the back to Reykjavik, then drive the south coast? Reykjavik more than an hour west of the Golden Circle so you will be doing A LOT of backtracking to go all the way east back to Vik.

Thank you so much! Yes, that's exactly what I was thinking but now realize it doesn't make sense after looking at the map. It seems like it makes more sense to have lunch in Selfoss, before continuing down to the South Coast. We'll have a child with us, so we don't plan on doing long hikes, but just want to stop and enjoy the sights for a while.

That doesn't sound like a very fun day to me, even with the long daylight hours. I'd do it in two days or consider overnighting somewhere like Selfoss or Hella.

We tried to do it and it's not enough time to do anything but drive. Break it up into 2 days and 3 if you want to get to Jokulsarlon for the icebergs.

I'm really glad I didn't listen to this thread! We ended up doing both in the same day and I'm really glad we did! It was a long day, over 14 hours, but totally worth it for us. We left Reykjavik at 7 AM did the Golden Circle up to Selfoss (it had maybe been around 6 hours at this point) and were trying to decide whether to go back to Reykjavik or keep on going as far as Reynisfjara Black Sand Beach. My wife and I were undecided, but our 8 year old son wanted to keep on going so we stopped at Bonus for some snacks before continuing on. I am so glad we did because we all absolutely loved Skógafoss and Seljalandsfoss waterfalls and the beach and we wouldn't have had the experience otherwise. My wife and I took turns driving so it wasn't bad and since it never got dark, leaving early and getting back lake wasn't a problem.

We never felt rushed, took our time at each of the sights and I feel like we were at most places much longer that most of the tourists and tour companies so we were really able to take everything in. Only thing I would have liked to have seen (other than from the road) was Solheimajokull - but we wouldn't have seen it at all anyway had we just done Golden Circle.