Belgium has complex housing and zoning problems caused by 50 years of inconsistent policies and regulations.
One (of many) big issue(s) is that the past has seen unchecked claiming of land for new projects leading to an acute shortage of land, while lots of existing houses and buildings are entirely outdated.
We are talking about houses which are 60 or 70 years old typically located in densely pouplated urban agglomerations and haven't seen little to no remodeling whatsoever during that time. These are hardly isolated, difficult to heat, are entirely outdated in terms of utilities and plumbing and so on.
The idea behind the difference is to incentivize people to buy and renovate older, existing houses and curbing the further occupation / use of free & open space with new construction and urban sprawl.
It should be noted: Belgium is also a country with an incredibly complex fiscal system. There are taxes as well as tax breaks on every aspect of life. Belgium is a complex federal nation with many levels of government. To put that in perspective: it's a country with 6 different governments. All of which turn politics and policy making, unironically, into 5D chess. The fiscal system tends to be the arena where everything comes together, generally as a compromise which is often more founded on pragmatism then necessity.
Historically, home ownership has been, and still is, massively incentivized in Belgium. But just like everywhere else, soaring housing prices and an ageing population are causing issues for younger generations who can't buy property without financial help from their parents.
And so, young people find themselves in a bind. They can't just buy any property, market dynamics and public policies push them towards buying outdated property only to face long, drawn out and expensive renovation projects just to bring everything up to date with modern, strict construction codes.