Kadastral revenue, area, ....notaries have a database with all prices and other information concerning the references with the tax office uses.
The Belgian tax office plays by different rules than the real estate market. One important rule is that if you estimate the property too low, you will have to pay a fine.
If the tax office doesn't accept the declared value, they will have to following in mind:
1. They have to be able to claim that the property is at least worth 1/8th more than what you declared;
2. They can propose a deal instead of going for an estimate by three independant experts;
3. If they don't accept you declarartion and opt for an estimate by 3 experts, they have to be sure the experts will estimate the property at least 1/8th above what you declared, because it's a blame for the tax officer if the experts say that your declaration was right, and he was wrong to ask for more.
So, if you declare a very low value, the tax office will offer you a deal, which will be that you have the pay tax on a very high value. They can do so, because they know that if you don't accept this very high value (and high tax), they can be confident to ask for the estimate by 3 experts. As you declared a very low value, the 3 experts will certainly give an estimate which is at least 1/8th higher than what you declared. The problem for the tax payer is that he will have to pay a fine in this case. So it is true that the 3 experts won't accept the very high value proposed by the tax office, but at the same time they will collect a fine on top of the inheritance tax on the property. So in the end you pay way too much tax and fines if you declare a too low value.