Extending My Trip By One Day, Your Experiences And Arguments Against It?

I’ve been looking so long for my vacation, going alone to Spain. However it’s been hellish with the temperatures lately, as it’s 31 celsius (think that would be 90F(?)), and it’s impossible to cool down. On the way back I am staying overnight at a Bed and Breakfast in France, I want to add another day here.

What are your experiences to extend your trip? In my case it’s just a day.