Almost all the "built in" ones are functionally the same as the "add on" ones, the main difference being the guarantee of good seat fit. In all cases the actua

Almost all the "built in" ones are functionally the same as the "add on" ones, the main difference being the guarantee of good seat fit. In all cases the actual toilet is made of porcelain, which as you might imagine does not lend itself well to installing motors and electronics. Even for units that are sold as a combo, the bidet is built into the seat/lid, which is exactly how the add-ons work. I installed one of these on an existing toilet in my house and it was fine: https://alphabidet.com/products/alpha-ix-pure-bidet-seat https://alphabidet.com/products/alpha-ix-pure-bidet-seat I've since swapped out to a different toilet that came with a built in bidet and it's also fine, not markedly better. If you're replacing the whole toilet might as well just get an all in one with the guaranteed fit. If you weren't going to, there's no reason not to just get the seat.