We used to work with a LMT who came in when a guest requested it (we had it listed on our website) but it wasn't requested often. We later had a consultant recommend that we turn a large sitting area in our B&B into a spa instead of having it done in the rooms and heavily promote it on our website. We would pay the LMT for their services and mark up the cost as we chose fit. I thought about it for one minute and decided it wasn't for us. Here's what made me say no so that you can consider it -- perhaps it will be right for you given that you'll have separate space for the LMT to enter and work and you have the wedding business.
1. Scheduling in the past has been tiresome for us. The guest tells us when they want the massage, I call the LMT and if not there she gets back to us (often they aren't getting back to you until after all their appointments). If the time isn't available, I have to get back in touch with the guest again to offer the available time. Often went back and forth with several calls/emails taking lots of my time. Our front door is always locked as well. We would then be responsible for staying here to meet the therapist and get her to the guest room. Since we need some time away, I didn't want another reason to have to hang around to let someone in.
2. The ROI on building out the spa rooms would take quite a while for us to recoup if we weren't charging a high price for the services like the large local hotel with a new super spa.
3. We are then subject to additional requirements/inspections for the spa space. Obviously we would keep it spotless but another thing I had to worry about.
4. Most of our guests are here for an outdoor activity, not really leisure relaxing guests. I think that's why it never really was requested often so know your guests.
5. Speak to your insurance agent about what additional coverage you would need.
6. Be sure you can find a good LMT who is responsible and will show up. The guest will have an overall negative opinion of their stay if their massage is cancelled. We live in an area where they tend to be transient and move to areas with more clientele paying hire prices as you don't want lots of turnover.
I'm sure spas are a good add-on/investment for some but for us we do everything ourselves pretty much the added work wasn't worth it to us..