Years of historically affordable mortgage rates could come to an end as rates rise and are expected to escalate even further throughout 2018. So far, mortgage rates have increased nearly 50 basis points since the beginning of this year, according to Zillow.

The real estate resource’s Q4 2017 affordability analysis found that typical mortgage payments around the country required 15.7 percent of the median income. In comparison, mortgage payments required 21 percent of the typical income during the late 1980s and 1990s.

Forecasters estimate that mortgage rates will increase around 6 percent next year, which will drive up monthly housing costs for the median U.S. home about 20.5 percent, nearly the historic average of 21 percent. However, monthly mortgage costs have already surpassed the historic average in seven large U.S. markets, including Portland, Denver and San Jose.

Higher mortgage rates will impact housing inventory as well, as some homeowners with lower rates may hesitate to sell their homes and take out a new home loan with a higher rate. Once rates begin to approach their historic levels, they will destabilize housing affordability for buyers, thus potentially limiting the number of homes on the market, according to Zillow.

The amount of income spent on mortgage payments in Boston is 23.5 percent, whereas historically, from 1985 and 2000, it was 26.2 percent.

“Rates are showing a clear upward trend, bringing an end to an era of historically affordable mortgage payments,” says Aaron Terrazas, senior economist at Zillow. “Bigger life considerations typically take precedence in the decision to move, but some homeowners who locked in a lower mortgage rate may look to alternatives like renovating their current home instead of becoming a buyer in a stressful, competitive market when higher rates would limit their buying power below what it was when they bought their current home.”