I've lived in Sweden and Iceland, both of which have pre-filled tax returns to be submitted either on paper or electronically. Sweden is particularly easy, all I have to do is to verify the numbers and then send an SMS with a personal code printed on my return. If I need to change anything, I log into the tax office website, make the changes and submit.
I have deductions to offset paid interest. If I hire a person to do some renovation in my home, the tax office pays half of the fee directly to them and I get a deduction as well. I sold my flat this year and will need to pay tax from the winnings, deducting costs. Almost all of this will be pre-filled.
In Iceland I had a slightly more complex tax situation. I filed jointly with my ex wife. Mortages, minimum wage deduction (i.e. you don't pay tax of the amount that is the legal minimum wage), property tax (house, cars), stocks and bonds, bank deposits and earned interest, deductions for rent compensation, was all prefilled.
Compared to this, filing my dead-simple US taxes with one deduction, filling in a two page calculation worksheet, feels like the middle ages.
No matter the complexity of the tax code, there are systems in place for all of them to evaluate and error-check tax declarations. The information is most likely all there already. All you need to have is a way to authenticate tax payers, and a front end.
And if I have a reason to think the tax authorities will not include items that lower my dues, I can always hire an accountant to file for me. I simply give them my "third-party filer access code" and sign the final return.
In Norway, if you qualify for simplified tax return (which I think most people do), you don't have to do anything anymore. You get a default pre-filled tax return that you can access electronically (or on paper if you prefer), and you only have to return it if you need to make changes.
Same in Denmark; you also get the refund automatically paid, if they owe you anything, with interest (0.5%, but still). I screwed up my withholding my first year here (failed to apply for a tax ID at all, so taxes were withheld by default at the highest rate). The next March I got a letter saying, your taxes were over-withheld by large_amount. If this is incorrect, please log in and fix it, otherwise we're going to deposit a huge pile of money in your account in N weeks. Did nothing for a few weeks and the money showed up.
To elaborate: In Sweden, even if you accept the defaults, your creditors and employers will send you statements (sv: kontrolluppgift) of your debts, interest, earnings, income et c that you can easily double-check against the defaults.
I have deductions to offset paid interest. If I hire a person to do some renovation in my home, the tax office pays half of the fee directly to them and I get a deduction as well. I sold my flat this year and will need to pay tax from the winnings, deducting costs. Almost all of this will be pre-filled.
In Iceland I had a slightly more complex tax situation. I filed jointly with my ex wife. Mortages, minimum wage deduction (i.e. you don't pay tax of the amount that is the legal minimum wage), property tax (house, cars), stocks and bonds, bank deposits and earned interest, deductions for rent compensation, was all prefilled.
Compared to this, filing my dead-simple US taxes with one deduction, filling in a two page calculation worksheet, feels like the middle ages.
No matter the complexity of the tax code, there are systems in place for all of them to evaluate and error-check tax declarations. The information is most likely all there already. All you need to have is a way to authenticate tax payers, and a front end.
And if I have a reason to think the tax authorities will not include items that lower my dues, I can always hire an accountant to file for me. I simply give them my "third-party filer access code" and sign the final return.