Gregory Goossens answers the "ask the expert" questions on the expatica.com website. This is where he works for money. His company deals with setting up expat status and I'm sure would be able to tell you the difference between income taxed at ordinary levels and expat levels if you can't continue with expat status.
www.taxpatria.be/pages/services.php
Here is some of the advice and information which Gregory Goossens has written for free on the expatica.com website on working out tax and social security implications. Don't forget Belgian salaries are usually quoted gross per month, after working a whole year under a Belgian contract, you usually get 13.92 months, an extra 1 month before Christmas and .92 in the Summer.
On a Belgian tax return, you can claw back tax paid on child care for under 12s, mortgage payments (including properties mortgaged abroad), house maintenance costs, all sorts of things.
www.expatica.com/be/finance_business/tax/Calculating-your-tax-burden-in-Belgium_15334.html
Here's a summary of the tax exemptions you get AT SOURCE, so long as you tell HR your family status, before you fill in your annual tax return, after which you can claim back the extras like child care and hopefully get a bit of a refund, NB if you've underpaid the tax, instead you'll get a bill!
You get first ?6070 or ?6800 tax free (if earning over 25,400)
You get ?1200 tax free for 1st child
You get ?2280 tax free for 2nd child
You get ?4610 tax free for 3rd child
You get ?5150 tax free for 4th and further child
Not sure what a non working spouse is, think it's ?6800 tax free
You add all these together to get your basic tax free salary.
For me it's 6070+1200+2280+4610+5150 = ?19310 tax free per year
You still pay social security, no idea at which point it kicks in, but you get an allowance where you pay no social security.
Here's another useful link.
www.capitaltaxconsulting.com/international-tax/belgium/belgian-income-tax/
Here is a basic tax calculator.
If there is a non working partner, untick the Conjoint/cohabitant légal avec revenu box.
Go to the Enfants non handicapés à charge box and choose the number of children you have.
In the Salaire mensuel box, put in quoted monthly gross salary.
Click on brut -> net and you'll get the basic monthly net salary, before you do your tax return.... some people claim back several KKKs on the annual tax return, others like people who have other income have several KKKs to pay after their annual tax return.
payroll.partena.be/web-simul.html