Well, the Courthouse is currently on the Île de la Cité. It's a pretty large complex of historic structures, some from the medieval era (it includes the Sainte-Chapelle and the Conciergerie), covering a third of the Île de la Cité.
These from Wiki might remind you of the site.
Although it's something large, everybody says it's no longer convenient the way it is. They need something different, modern, maybe even larger and easier to maintain. So the new thing will be located at les Batignolles that's in the 17th arrondissement on the edge of the city, neighboring the suburban town of Clichy. It's not a bad neighborhood, but people who've worked in the old Cité for long will have the blues for a while.
In fact, from looking at the renderings, people have been raising the question to know whether a high-rise building is proper to a Courthouse. That's no regular office building at all, there are some peculiar functional constraints that explain the awkward massing. However (and thankfully), the building won't sit there as a massive fence between Paris and its neighboring suburb. Its orientation will make it easier to look at from Clichy and from the city center, whereas people living on either side of it will have to deal with the huge bulk in their views. Quite strangely, no one would complain in the neighborhood, they're okay with it. Voilà.
: I forgot to say what's in the courthouse, which would have made the actual needs clearer to myself.
The courthouse of the Île de la Cité includes:
- The Court of Cassation
that's the highest national jurisdiction, like a counterpart of the US Supreme Court to some extent.
- The Court of Appeal of Paris that's a regional jurisdiction, still quite busy, needs more room.
- The superior courts (Tribunal de Grande Instance) of Paris that's only communal and yet the busiest jurisdiction of the entire country, needs more room.
- The bar association headquarters.
A few other things, some related services. The police division assigned to the judicial services isn't within the courthouse complex proper, but in a neighboring building.
So only the superior courts and the police will move from the Cité to the Batignolles. They'll be joined there by the lower courts that have been scattered all over the city so far, that is to say each of 20 arrondissements has a lower court that will move to the Batignolles.
The Court of Cassation and the Court of Appeal will actually remain in the Cité's old courthouse. They'll be joined by the courts in charge of the employment law (Conseil de prud'hommes) and the regional annex of the National School for the Judiciary based in Bordeaux.
This is all about making more room to the various jurisdictions that have been cramped. We've heard of that situation for long in the media from time to time, it's nothing new.