Planning the flower bed plantings etc. is definitely the fun part. Most people skip to that step because it is the part of the landscape you see- and the most enjoyable part to think about. It's a classic mistake to do so because you will pay a long term price for all the seemingly little details you skip in the beginning. I take it that the advice of a good engineer feels out of the budget for you? Drainage issues are the boring stuff but it's already obvious you're going to have a significant drainage concern. Trenching and piping are okay but eventually trenches can fill with sediment and no longer work. You really, really need to have an engineering consult to determine if what the builder proposes will be adequate. It shouldn't cost more than a couple hundred bucks at most. Perhaps some other neighbors will pitch in since you'll all need the same thing? I used to be a City Planner. I've seen developers pull a lot of crap. Sometimes, they are well-intended but the data they based their information on was faulty. Other times- they're just out to make a buck as quickly and cheaply as possible. At an absolute minimum, call your City offices and see if someone from the City engineering Dept. can advise you on appropriate methods to handle the drainage. In theory, they've already required the developer to deal with this but sometimes really different to say it will work on paper and to see it once everything is going in. Building codes are the 'minium acceptable standard', they are intended to set the bottom threshold. Something can be up to 'code' but not necessarily be a great solution. Also, ask them what level of storm they design for. Sometimes they require them to design the drainage to handle the runoff from a 10, 50, or 100 year storm. You'll want to know which they planned for in your case as it will give you a better idea of what it will handle. As for your other issues- a patio will be much lower maintenance over time than a deck and will certainly last longer. I would also make sure to have a 2' high by at least 3' deep berm for flowers and shrubs against the house as sort of a 'back up' in case your drainage system clogs or is overwhelmed.

If that photo is representative of the area in question, then I'd reconsider using this extremely tight space for "outdoor living". And unless I am missing something, that slope is not very severe. Even if leveled, I doubt this area would be congenial to lounging outdoors or utilizing as a patio, etc. Too exposed and too restrictive and without sufficient space to develop any kind of green privacy screening. I'd consider just a mixed shrub buffer between you and the neighboring property, perhaps with a meandering pathway connecting it to more usable areas of your property. Unless that photo doesn't reveal the true slope of the area, I'd say no terracing or retaining walls are necessary or even practical. Can you post photos of other areas of your garden that may be more conducive to outdoor living/entertaining? I still think this is a situation where engaging a professional for an hour or two can be helpful - it is difficult even from the photo to get a good visual and a true sense of exactly what you are dealing with.

Just because a website says a plant should do well in a given area doesn't mean that is necessarily will. All information must be taken with a grain of salt and then verified before one invests too much. I'm sure the original design intent is that the junipers grow together and be the "groundcover." Using mass shrubs as groundcover has the disadvantage that it's difficult to limit their height without mucho maintenance. What inevitably ends up happening is that they become too large and unwieldy, make somebody unhappy and then get ripped out and replaced. Adding another groundcover in between them would look weedy. Though I would do this if you intended to replace them with that other groundcover, and remove the juniper as the other groundcover took hold. Another disadvantage to the sea of junipers is that you really can't add trees (though it looks like some might be useful for screening) because of the shade they produce. Plants that demand full sun It would help on the weeds if you used Preen, a pre-emergent herbicide that attacks germinating seeds, and mulch heavily. Many people give up on Preen because it usually doesn't do a great job until after its second application. And also, because they don't adhere to the schedule, putting the second app. on way too late so that it becomes just another "first" application. I have found it to be effective and basically a life-saver. It can cost a fair amount to cover a large area, but in my opinion it still beats the total cost of hand weeding once the time and PITA factor is added up.

I would definately look at terracing the slope for a lot more useable yard. You can still use the work idea from above. We had a very sloped hill next to our house the water ran straight into the basement of our house before we bought it so the walk out of 1600sq ft had never been developed not even windows. The first thing we did was dig away from the foundation and take all that material wawy to form a hill with a good retaining wall with gravel. big o and move all that water away from the house . In 13 yrs we have never had water in our now nicely finshed walk out basement .The hill that was formed from all the dirt now has a pond and lots of plantings and looks amazing . The retaining wall provided a really nice private space for outdoor dining right next to an outdoor kitchen . Think of what you can do with some terracing.