What would happen to the Royal Trust and the palaces if Belgium abolishes its monarchy?
The Royal Castle of Laeken and the Royal Palace of Brussels are actually owned by the Belgian state and, presumably, would continue to be so. The Royal Trust, which includes other residences such as Ciergnon, Fenffe, Stuyvenberg, Belvédère, as well as Astrid's and Laurent's villas and the greenhouses and the park of the Castle of Laeken, is a different and more complex matter.
When King Leopold II transferred his private assets to the Trust, his donation was explicitly conditional on three terms: that the land and buildings would never be sold, that they would have to retain their function and appearance, and that they would remain at the disposal of the successors to the Belgian throne. If Belgium becomes a republic, there won't be a throne to succeed to, but I suppose the royal family could make a legal case that that they are perpetually entitled to the right of disposal over the Trust assets under the terms of the donation. It remains to be seen whether they would succeed in court or not.
Anyway, your question is purely speculative, as was a similar question asked in the British forums about the future of the revenue from the Crown estate and the Duchies of Lancaster and Cornwall if Britain became a republic. Nobody can know for sure in my humble opinion.
EDIT: There is a page on the Belgian monarchy site (click on this link ), which mentions the three conditions above and also provides further information on the Royal Trust.
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