Research the market thoroughly! Look at the level of hotel (star rating, average cost per night to stay, what is the average resident spend on a stay) and compare this to other hotel spa's of a similar level.
What type of people stay at the hotel? What do people visit the hotel / area for? What is the percentage of male and female guests. This is really important to understand to ensure there is a big enough market for you there, and that you tailour the treatments appropriatly. We recently stayed at Gleneagles and visited the Espa spa (their UK flagship) there. Talking to the therapists they have very few male clients - British men do not do Spa's as a rule. What is the percentage of nationalities that stay, different countries have different attitudes to Spa's, treatments and products. Understanding as much about your potential market who will be very easy to market to will really help you build the client base.
Examine the space thoroughly, take pictures and measurements of all the spaces and rooms! Are the treatment rooms big enough - are their enough? Is there a relaxation area away from reception, are the boys and girls changing rooms big enough? Do the changing rooms have enough space in them for enough hair dryers for the girls to use? How do other spa's in the treatment price range layouts compare to this one?
Does the Spa have heat treatments? Sauna, Steam Room, Vitality Pool etc - look at the upkeep of these and running costs.
What product ranges would be suitable for your guests? And this is a really really important one to get right! And this is very dependent on the type of guests you will be looking at targetting - are you looking at a good reliable skincare brand, something that has name appeal like Dermalogica or are you looking at something really top of the market like Espa (my personal favourite)?
When looking at the product range to use you also have to think about the clients, a lot of whom won't be regular customers - its a hotel so by its very nature a high turn over of potential clients. Would they want to buy something like Dermalogica from you when they know they can get that on their local high street? Or would they want to spend the money on a product that isn't as available like Espa for that exclusive feel?
You say the Spa is part of the hotel, you have to be very clear about boundaries of responsibility on this. How much investment will they make into the area / services ... is maintenance of fixtures and fittings included in your rental agreement? Is maintenance of the showers / hot water / electrics? How about cleaners, will the hotel cleaners do that? What about your laundry costs ... do you need to arrange your own or will you be able to use the hotels and at what cost? What about heating and electricity - how is that paid for?
Are any employees of the Spa employees of you or the hotel? There needs to be clear understandings on responsibility for staff, but also complaints and enquiries. If the hotel switchboard takes a spa enquiry who do they pass that onto, what happens if they're is no one available?
Also discuss marketing the Spa - are you able to use the hotels name / branding? Will the hotel put a leaflet / flyer in each room as part of the day housekeeping standard routine? Will the hotel feature the Spa in their marketing materials with a "see the website ..." etc.
Discuss insurance requirements, will the hotels liability insurance cover you or should you have your own? Don't forget employee liability if the staff are employees of yours. What about fire regulations and health & safety? Responsibility for these kind of things needs to be agreed and be part of the rental contract.
Also don't forget when things are progressing and you are negotiating contract terms to ensure all of these points are covered in the contract. Make sure you get a good lawyer or legal representation (Federation of Small Businesses membership can be an excellent value for money way of getting this).
When I stay in a hotel and use their Spa I always expect to have any treatment / product costs added to my room bill. How will this work for you? What are their procedures against a customer who's card is declined for payment upon checkout - how will this affect you?
Hope this doesn't sound too negative, but its the details and covering yourself now that will really pay off if the venture goes ahead!
I love hotel Spa's, I've visited some very average ones and some of the best in the country and if the hotel is busy enough with the right type of guests, and you design and manage the Spa and the treatments well enough it can be a great business to be in!
Good luck! Please if you have any questions about any of this just give me a shout, more then happy to help out anyway I can.
Mat