...it existed when I was there and it still exists today. So is it is or was?
Both are correct so long as you're consistent. Right? It's a style/mood thing, right? Right?
I'd say that even though you are right on the last two points (consistency and style/mood), you are seldom right on the first (it still exists today, therefore present). Why seldom? Because there may be occasions (rare, I believe) when you can pull it off on those grounds.
The way I see it, narration should be written in the past.
[yes, I know some stories are written in the Present, but that is harder to pull off and the last ones I did read in that tense didn't work for me at all so... Anyway, 'shouldn't' is never to be mistaken with 'mustn't'.]
If one is writing the narration in the past, using the present will always force the reader into a different shift. That means that it must be used ocasionally and for creating the right effect.
[Do note I'm talking about narration, not dialogue.]
So when to use it?
1. For the narrator to address the reader directly
I lived near Les Gobelins. If you've ever been to that area, you know it is a beautiful intersection full of trees.
This works best when the narrator is in the 1st person, but it can also be pulled off in the 3rd person, albeit it will require more skill.
She lived near Les Gobelins. If you've ever been to that area, you know it is a beautiful intersection full of trees.
Even though the story is in the past (which creates some distance from the 'now' that is the reader's life), if the narrator talks to the readers, it will pull them in. Using the present bridges the gap between past events and the present of the readers. Moreover, the past underlines the story's eventual fictionality; using the present underlines that at least that snippet of information is real. Or 'more real' than the rest of the fiction.
2. To hear the character's thoughts
I walked briskly towards my house, near Les Gobelins. I'd lived there all my life, hoping one day to go away and see the world, and now, after a year living abroad, I looked at that intersection full of trees and stopped. God! Isn't this the most beautiful place on Earth?
If done well, we can drop tags such as 'I thought' or 'she said' and give the thought/line directly, because the present tense will be hint enough this is the character's voice. Of course this approach doesn't mean we write off those tags for good; it just means we can choose between how deep inside the character's head we are.
3. To reinforce you're conveying facts
The house my aunt had rented for me was near Les Gobelins. It wasn't far from the bus stop so I got the map out and started navigating the streets. The first view I had of the area took my breath away. It is this beautiful intersection full of trees and, at that time of the day, the sun seemed to create a halo of sacred mysticism over it. I was in love.
I'd still avoid more than one or two verbs in the present and I would definitely 'sandwich' it with the overwhelming past. You'll want to jab the reader slightly, not jar him out of the tale.
Remember that the story is in the past so bringing in the present basically says 'break time: listen to me preach about whatever'. In this particular example, because the break is so short, it ends up pairing with the previous phrase ('...took my breath away') to help the reader stop with the character and say 'wow'. And then the tale continues in the past.
4. To create a jarring effect
I re-used the excerpt from point 2 for the first paragraph in order to create the right context for the final punch.
I walked briskly towards my house, near Les Gobelins. I'd lived there all my life, hoping one day to go away and see the world, and now, after a year living abroad, I looked at that intersection full of trees and stopped. God! Isn't this the most beautiful place on Earth?
A little voice sneered I couldn't possibly be comparing that mess to Versailles or New York, but my heart wouldn't be fooled.
I am home.
First of all, this is still the character's thoughts. It doesn't change the fact it has a jarring effect in this example.
Imagine this is the end of a chapter, a section or even the story. You went through it all in the past (gone, distant) and all of a sudden you force the reader into the present (now, immediate, near). By doing so you create a movement 'past > present' and, in this case, when you stop abruptly, it's not unlikely the reader will continue that movement 'present > future' and expect the character-narrator to choose to live near Les Gobelins for the rest of her/his life.
To conclude, you can use the present amidst the narration in the past but don't do it because the facts you're relaying are still true now. Do it to create the right effect in the reader.