Can a 501c3 solicit donations for the purpose of improving property that does not belong to the 501c3? For example, a non profit that has raised money to build an addition to an existing building that is not owned by the non profit and is not on land owned by the non profit, but that the non profit leases .

It's possible for a 501c3 to raise money that they in turn donate to another 501c3. But if the property in question is leased from an individual or a for-profit business then this would only make sense if the 501c3 gets good economic benefit in return. For example the improvements they pay for could be in lieue of their regular lease payments, or if leasing a different property that already has these improvements in place would cost more in the long run then it might make sense. However they ought to have a very tight legal contract with the owner so that they don't end up paying for an improvement and then losing the lease.