In Brussels, the capital growth for appartments has been +/- 10 % for the last 10 years. Now the growth may slow down a bit for the next years to come - but overall is Brussels not expensive, compared to other capitals.
If it's a new build you are looking at, be careful to chose the right neighbourhood and quality.
Avoid the city center; people who have money live uptown ("haut de la ville").
Locations that are good to invest:
-Everything in neighbourhood of the CE buildings (with all the new countries joining the CE, there's more and more demand for appartments nearby).
-Ixelles and some parts of Saint-Gilles (Neighbourhood of Place de Chatalain / Rue Americain and a bit further Altitude Cent).
-Personally I don't like the "Quartier du Nord" (neighbourhood around the North Station) - for years it was the worst neighbourhood of the city but with the CE closeby and all the new offices, it is a good investment for sure.
If you want to invest in Belgium, rather than in Brussels - there are a few other cities to consider, where the real price boom still needs to take off.
Gent and Leuven are much cooler and safer cities, and a better investment in the years to come. And at the coast there are Nieuwpoort-Stad, De Haan and Bredene, where prices are expected to take off spectacular in a few years.
But if you have a specific project in mind, please let me know the neighbourhood and I can tell you what th pro's/contra's are.