What are the installation requirements for the unit?

I would prefer it not to be seen. It does not have to be IN the bathroom itself to provide you with the amount of hot water which will be used mostly by the shower. You will need to place it within a certain amount of distance depending on the specs for the heater. Is there a closet near etc?

In Europe they are often above the toilet to make the distance for use as close as possible , The unit you choose will have install instructions and the plumber follows those . I am not sure how that placement restricts access . As for another place I would need to see your plan.

In many places the tankless water heater units are placed outside on the exterior wall closest to the bathroom.

Ours are in the garage, because our HOA was confused at the time we built our house ... they now allow them outside.

We have not purchased yet only done some research on size and viability. We are still in the schematic design phase of house plans. This is the master bath and toilet is somewhat hidden. Outside walls are strawbale house (this is our second straw build) and there is no natural gas, it will be electric water heater. Also we are at 7300ft elevation in Colorado. I do not want in my clothes closet . I have another option in the breezeway to this 'en suite' from the main house but I thought it seemed practical above toilet as nothing really goes there anyway other than for aesthetics.

I would think on an outside wall in Colorado would be very weird and not practical . They need to be protected from freezing so no to outside . If you post a pic of the actual one maybe we can help further. Most of the ones in Europe have a lot of the stuff in the wall some with access for repars. Neither of these are all thta ugly and could easily be above the toilet .There are many that do not have to have all that exposed but the better solution is to have all the hot water needing items on one side of the sape too not for pipes funning all over . That probably why they work so well in the European small bathrooms

I wouldn't want it mounted over the toilet would you?

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I think visually, it's not great over the toilet. Can you make the left wall of the shower thicker - taking space from inside the shower and make a recess in the wall on the toilet side to put the heater in there? ( obviously depending how thick they are ). It also looks like you have some space alongside your sinks - can it fit in there?

Okay folks, let's back up... This is probably going to be an electric - not a gas - water heater. Those photos posted by Beverly are not what you would be looking at.

I would suggest that you use a small electric TANK water heater and install it in the sink base. It would be in series with the main water heater so it only has to produce enough hot water until the main unit takes over (probably a couple of gallons). Here is a link to a 6 gal unit that would fit under a sink. This is 120 volt, 20 amp circuit - so pretty straightforward electric.

Make sure your hot water lines are insulated! You don't want the heat from the main unit to dissipate to the slab, for example, before it ever gets to your bathroom.

Alternatively, you can consider a recirculating unit that keep the water warm all the time. This too must be well insulated AND have a dedicated, insulated, return.

We were thinking of doing a recirculating loop as well as the extra tank just because it is so far away. I really don't want to have problems down the road and wish we had done this during planning. It would have to be electric because of our location - no gas (could pull in propane but not a huge fan of that). But Pegasus you may have a point of whether or not this is really needed. Thanks maybe this is too much for little gain after all.

Appreciate the discussion from all of you!

For your main heater, consider a hybrid electric. Definitely don't go with whole house Tankless electric.

Thanks Jake. We are thinking of a heat pump water heater for the main house with a heat pump. But because our bathroom will be about ~95' away from this, we thought about the smaller tankless for it. Our designer mentioned the recirculator but he didn't know much about how it worked exactly so we've been doing the research. I wanted to make sure we planned as needed for the right spot . I like the under sink if we needed.

Thanks Jake. We are considering a heat pump water heater for the main house with the heat pump.

I get where you’re coming from about wanting to use every bit of space, especially when designing a tight bathroom. Personally, I wouldn’t put a tankless water heater over the toilet either, though, it sounds okay in theory, but in practice, it’s pretty inconvenient if you ever need to access it for repairs or maintenance. Crawling over a toilet to check connections or do a reset sounds like a pain. In my last reno we mounted one under a vanity cabinet, and it worked out fine with just enough room to get to everything when needed.