We are in the same boat as you guys. We do only high end residential-only work. All phases of design/build/hardscape/landscape/maintenance/lighting, etc. We've successfully made the transition from doing a little lighting to being one of the biggest lighting installers in our area, even compared to the lighting-only companies. It's a big leap to go from the one level to the next.
First, stick to your guns. People like your company for all the other stuff you do for lots of reasons, right? But not usually because you're the cheapest. Usually you get jobs because of quality, reputation, portfolio, word of mouth, etc. - right? So lighting is no different. You've landed big landscape and hardscape jobs even though you were the highest bid, right? So you can do the same thing with lighting. The only difference is you maybe don't yet feel quite as confident and aren't quite as experienced in the lighting category as you are in the other categories, am I right? At least that's how I felt. I really wanted to get a lot more lighting work but was struggling with it, not landing a lot of lighting jobs and it was a catch-22. How was I ever going to get good at it if I couldn't land many jobs. Fast forward 7 years, now I can land big lighting jobs no problem. It's a big part of our business these days, where it used to be a very small part. What's changed? A few things....
1) I got a lot better at taking photos of our outdoor lighting work. That's made a huge difference. Once we had as nice of a portfolio of lighting work as we did for paver patios, outdoor living areas, etc. then it started to become a lot more easy to sell lighting jobs. Getting good at the photography is a huge challenge. Plenty of posts here in this forum about it. But get good at this. This sells jobs more than anything else - having great photos of your work.
2) Get to really know lighting in and out. Learn everything you can about the different brands that are available at your local distributors. Go to all their classes. If they invite you for a factory tour, go on it! Learn from this forum. Get the books that are recommended in the threads we've had in this forum. The Janet Moyer book, the Nate Mullen book, et. al. Get really into lighting and design and start learning what the big lighting pros have learned. This knowledge will help you make good decisions and feel a lot more confident with your customer.
3) Promote lighting. We market lighting just as much as we market anything else we do. We upsell it on every hardscape/landscape install. We do post-cards and other marketing promotions for just lighting. We have internet marketing set up for just lighting. We promote it heavily. We promote our lighting on our Facebook page, Twitter page, etc.
4) Create reasons why the customer would want to pick your company over the others. Know those reasons up and down. I go into most lighting jobs assuming I'm going to be the highest bid and so I start from the very beginning explaining that our lighting jobs aren't cheap but here's what makes our systems different. Here's what's worth paying more money for. Here's how it's going to save you money vs. what the other guys are going to try to sell you. I'm pre-empting any of the issues BEFORE they come up.
5) Separate yourself as a lighting designer / expert - not just another lighting installer. This is hard to quantify and hard to learn. But there are plenty of threads here in this forum about this.
If you want to talk more, PM me. I'd be happy to help. I love doing lighting. Even though we do a lot of landscape/hardscape stuff, lighting is one of my favorite things. One of my passions. We could bounce ideas off one another and I'd be happy to share some of my other secrets that I won't post openly in the forum that have helped us in lighting. There aren't many of us full scale landscape companies in this forum. Most of the guys here in the forum are lighting-only and, I know I'll get in trouble for saying this, but some of them are a little antagonistic to guys like you and me. There is a line of thinking that some in this forum have that guys like you and me who do all phases of landscaping will never be at the same level as the true lighting gurus here. So their comments are sometimes more antagonistic than helpful. Maybe lighting guys feel threatened by lawn care/landscapers stealing part of their industry. I guess that's a valid thought. Anyway, I'm just saying I'm happy to help. Been there and I understand the transition you're trying to go through.