Otherwise, I'd say drainage is always a major concern when dealing with a very flat site, or one that is steep. In both cases, drainage is necessary to either take it away, or manage it so that it doesn't cause erosion and/or flooding. At least here in most of California, drainage is mostly just a late fall into late spring concern, as the rest of the year there is usually no rainfall at all. But it never pays to ignore drainage in the initial design, but to plan for it, especially if you see visible drainage problems before you start to redesign the landscape.

I'd think drainage tiles are really only a concern with playing fields that are almost flat and have poorly draining soil types. They still get installed for such locations because it is usually important to be able to use the play fields immediately, without having to wait for a slow draining grass field to become dry enough to allow use without damaging the turf or overly compacting it. This usually isn't a critical concern for a private residence.

I see a lot of poorly planned and/or executed walkout basements that cause either a low area with no drainage outlet or erosive runoff. But another big problem is that a number of architects believe that gutters detract from the beauty of a home and refuse to have them on the house. Add a few jogs and a walkout basement and you have potential for too much groundwater on the high side of a basement wall.

Large areas of hardscape shed off 100% of the rain that lands on them rather than absorbing it. This is often in one direction (away from a structure)which concentrates more water in one place in creasing the volume of water. Another result of creating large areas of hardscape is that they often require added fill to level out the area. Many times this creates a steeper slope adjacent to the hardscape and on the side that the water is pitched toward (away from the structure). A steep slope equals faster runoff. Erosion is caused by volume and speed of water, both of which are enhanced by the description above.

Most of the soils where I am are excessively well drained (large grainy sand), so it takes water in very well for the most part. But it also gives it up easy to things like basements. Drainage swales are often very effective here as well as drywells or drain tile.

I also have water that pours out of my gutters, down the pipe and straight to my foundation & have bought 3 rain barrels to collect the water & when they arrive will use a soaker hose to my flower beds during the times where rain is lacking.

I'm not positive who said this, but I believe it was Pat Morita.

Since we've already diverted the greywater to the pecan orchard, and added new lines, all that's left is major dirtwork. We've contemplated replacing the original swale, and building retaining walls along with digging another swale further uphill to divert more water before it runs down into the yard, but hadn't done anything for fear of damaging the beautiful, mature Elms on that side of the house. A few years ago, I set 150 feet of 18" flashing to divert the worst of it away from the septic field, but we held off doing anything more. To us, it was worth putting up with a temporary inconvenience for the sake of those trees.

Unfortunately, thanks to the ice storm this past weekend, we no longer have beautiful Elms, (just huge trunks minus 2/3rds of the original crown) so it looks like we're free to landscape and terrace at will. On the bright side, in addition to correcting the drainage issues, it looks like now is the time to add that walk in storm cellar and earth bermed greenhouse I've been dreaming about.

In answer to your original question Ink, for us the ideal time to look at drainage is when the opportunity presents itself. :^/