While middle-class families have seen their incomes stagnate, the rich have been getting richer. Both the well-off and the wealthy have experienced substantial increases in assets over the past several years. For interior designers looking to market their services to these wealthy clients, some recent reports offer valuable insights.

Despite an overall weak economy, the number of households with NIPR (not including primary residence) assets of $1 million or more reached a record high of 9.6 million in 2013, according to the Spectrem Group's 2014 Affluent Market Insights Report. That's 600,000 more than in 2012.

NIPR households of $5 million or more increased 100,000 to 1.24 million, and ultra-rich households (those worth $25 million or more) grew an additional 15,000 to 132,000. Those households that Spectrem labels the "mass affluent" (with NIPR assets of between $100,000 and $1 million) also reached a record high of 38.6 million, up from 37.4 million in 2012.

Much of this growth can be attributed to the rebound in the stock market in 2013. Wealthy investors held onto and grew their investments during the slump and benefited when prices again began to rise. The phenomenal success of high-tech IPOs, such as Google and Twitter, and buyouts, like Instagram, also has added thousands to the ranks of the rich.

Entrepreneurs along with doctors, lawyers and senior corporate executives make up the majority of the ultra-rich. Spectrem calculates that nearly half (46 percent) of these households have annual incomes of between $1 million and $1.9 million, and about a fourth each have annual incomes of between $2 million and $2.9 million (27 percent) and $3 million to $5 million (28 percent).

Baby boomers comprise almost half of all millionaires in the U.S., when the value of the primary residence is included, according to the Shullman Research Center's Luxury, Affluence and Wealth Pulse. Surprisingly, given the media attention to millennials' employment and financial woes, they boast the next-highest count of millionaires, 5 million, followed closely by GenXers at 4 million, with members of the Silent Generation making up 1 million.

This influx of wealth should bode well for interior designers. Demand for luxury goods and services is growing at a rate of around 10 percent per year, according to the Luxury Institute. The American Society of Interior Designers has projected a 4 percent increase in residential design services in 2014.

But today’s millionaires have not forgotten 2008, and they have shown themselves by and large to be financially conservative. Spectrem notes that wealthy investors are choosing to increase their portfolios and to decrease their cash holdings.

The Shullman poll finds that wealthy boomers are concerned about having enough money in retirement, Gen Xers are saving and investing to ensure their future financial independence, and millennials are saving for the day they start a family and purchase their first home.

When approaching prospective wealthy clients, keep in mind that their financial assets may be heavily invested in their portfolios or businesses, and their discretionary income may be less than you might suppose. They have money to spend, but they are careful or choosy in how they spend it.

Take into consideration their life stage and long-term goals, as well as their immediate situation. Remember, they place a high value on value and ROI, so be prepared to show them how your professional services will be a sound use of their money and provide dividends for years to come.