As long as you plan on closing it down for the winter, otherwise the pipes could freeze and burst.

I think that you should take the advice of your plumber and contractor over the word of anonymous people on the internet.

Then maybe you should quit giving it?
An actual shower needs a pressure balancing valve. These can be a problem in freezing weather and draining it for the winter relies too much on ones memory. Putting a hot and cold frost proof sillcock outside may be better. The objective of the shower may be helpful for a good response. The second floor porch sounds like it would be more for fun than utility reasons. Where do you intend to rout the grey water? Attaching an outdoor drain to the sewer system isn't good. By the time you are done, the shower would essentially be enclosed for privacy anyway?

I can't quit something I haven't done, Fred S.

Advice on who's advice to take is still advice. And telling someone what you think they should do is advice.
http://en.m.wikipedia.org/wiki/Advice_(opinion)

I'm picking up on some crankiness on tonight's posts and not just this one. Let's kiss and make-up, or get a good night's sleep? Grobby's advice! Lol.

It would scare me to go ahead with something my contractor and plumber both advised against. You surely wouldn't want to tell them to do it anyway, if something went wrong, they'd just say 'we told you it wouldn't work'. So, if you're determined to still try it, see if you can get other professionals on-site, who feel confident about it and will guarantee their work.

Fred- that was handymam's advice, to take the advice from the plumber and contractor. That doesn't mean she should quit giving advice, for pitysake. Some things should not be solved by a handful of unknown people on Houzz. That is my advice;)

I live on Cape Cod which gets long periods of freezing temperatures. I've had an outdoor shower for many years and there is an annual process of maintaining the shower. First of all there should be shutoff valves inside the home with built in drain cocks. The process involves shutting off these H & C water valves inside the home and then draining the remaining water in the outdoor plumbing lines through the drain cocks on the valves. It's a good idea to locate these valves in an open area to get a bucket under them when draining. This will get most of the water out of the exterior lines and prevent freeze damage to the pipes. Next, the outdoor anti-scald mixing valve needs to be disassembled and the shower cartridge removed. This is easier if the plumber lubricates the cartridge cylinder when the mixing valve is installed. The cartridge and all associated parts should be stored inside during the winter where they will not freeze. I keep them in a plastic bag near the indoor valves. I also include instructions available online in the bag so I can remember how to put it back together in the spring. You should always lubricate the cylinder when reassembling the valve. This is an annual ritual that takes about 1 hour. It's well worth the advantage of having an outdoor shower.

P,S,. If your plumber doesn't understand this, maybe you should get a second opinion.

I have been a Designer/Builder in Bethany Beach Delaware for 30 years. Every house I build has one or more outside showers. For easy winterizing I install shut off valves at the nearest hot and cold water supply under vanity or kitchen cabinets or of course at the hot water heater if it is near the outside shower. Good luck with you project. Please follow me on houzz.com.

An all weather outdoor shower is easily done by locating the mixing valve inside the house and use a coupling shaft to locate the handle on the outside. You must use a standard tub type lower faucet (with the pull up to activate the shower) to allow the system to drain out when the shower is done. You also have to make sure everything slopes in direction of the water flow. We live in Bancroft Ontario so winters are well below freezing and we have been showering outside for 3 winters. Only issue we have is the shower head will freeze so we typically remove it after a shower or shower without it on when really cold. The system has worked well and it helps that nothing special needs to be done by anyone. no inside valves to close etc. Just turn off tap and everything drains.

Thanks Mike, do you have a photo of the coupling shaft or a diagram. Your solution is close to what I had in visioned, as we have a frost protected valve for our garden hose and can turn the outside tap on during the winter for the rink. I was planning on putting the mixing valve on the inside at a present temp and then turning on the outside shower with one frost protected tap. I like the tub faucet drain detail but would like to keep the shower head on during the winter if possible. I will try to find a shower head that easily drains.

Every beach house I built at the Delaware shore has an outside shower. I’ve built houses with three outdoor showers. There is absolutely no reason not to have an outside shower. We set our clients up with an easy way to winterize their outside shower.

Because I could find no plans for an outdoor shower to service my hot tub during our cold winters in Ky., I designed my own. No issues so far. No need to winterize. That's when we use it! Any pipe material would work. I chose brass. I used common exterior hot and cold valves that self drain when they are closed thru the bottom ball valve which also doubles as a foot wash when shower valve is closed.

Vic do you have a better picture for the bottom ball valve?

For showering when it's freezing outside, wrap the pipes with heat tape (e.g. Frost King 30 ft. Automatic Electric Heat Cable Kit-HC30A ) and plug it in. No need to drain. Showerhead could freeze, so make sure the heat tape covers the showerhead too. Use freeze proof valves (e.g. Everbilt1/2 in. x 10 in. Brass MPT x MHT Anti-Siphon Sillcock Valve) and wrap them with heat tape too.

Where do you drain to?

Downspouts connect directly to the sanitary system, so I just installed a T to the downspout. The water is warm and freely drains to below the frost