I live in Scandinavia and that's how it works here: you basically get a proposal based on what the tax office knows (salaries, local capital gains/losses, interest deductions if you have mortgage etc.), and then you submit diffs, if any, and they calculate everything based on that.
I've only ever had to change my tax proposal when I've married/divorced/had kids or had activity in a brokerage firm abroad (who wouldn't report my gains directly to my local authorities). Buying and selling property also shows up automatically as the registration process goes through the government anyway.
Now the tax authorities have even stopped sending any papers with a return envelope. They just point to the online service where you can easily augment whatever fields that might be missing input.