You get the 12 mile military and 200 mile fishing limits for your land per international law. However, this must be land above the water. You cannot find land under the surface, dump tons of dirt on it, and claim those rights, per same law.
This doesn't mean you can't create the islands, but you can't do the 12 mile/200 mile thing. China [a nuclear power with a massive army and permanent UN veto]