No matter what you call it, an in-law suite or an intergeneration living unit, it's the same thing in the end: one address, two dwellings. Post-recession, these additions are back in vogue. Before you proceed toward creating a place where someone can live next to you-but not with you-don't forget these considerations.
Is it legal?
To relieve the housing crunch during and after the Second World War, many single-family residences were divided in two. Succeeding decades saw the rise of zoning; under these laws, existing two-family residences were allowed-they were 'grandfathered' in-but restrictions were put in place to limit their construction.
So the fact that you might see in-law suites around your neighborhood does not necessarily mean you can add one of your own. Consult with local building officials to find out exactly what you can and cannot do along these lines. Handicapped access or additional off-street parking may be required for compliance with current regulations.
How will it affect cash flow?
If you're planning to rent the in-law suite, it may be possible to write off some of the improvements or repairs associated with making the space habitable. You might even be able to get a tax break on some of your in-law suite's operating expenses (e.g., a portion of the water bill).
But don't get too excited: Besides routine maintenance costs, you might also have to upgrade your homeowners' insurance and/or shell out for scheduled safety inspections. Calculate both the expenses and the income you anticipate and as always, confer with your accountant.
Will it annoy neighbors?
Sure, an in-law suite probably only means one more car on the block, one more person coming and going, but in dense areas, neighbors are often annoyed by any disturbance to the status quo. Weigh the potential impact of your in-law suite on the houses nearby and if possible, build in a way that promotes privacy and shields your occupant's presence from plain view.