The Federal Emergency Management Agency’s (FEMA) recalculation of flood insurance premiums goes into effect Oct. 1, which will have a significant impact on homes in coastal cities and in flood-prone areas.

FEMA oversees the National Flood Insurance Program, which is the most widely used flood insurance program in the country. The new Risk Rating 2.0 has received accolades, as it more accurately reflects the real costs of flooding, but it will also mean rate adjustments for an estimated 5 million policyholders.

Some of those people will see their rates shrink, but some areas will see steep increases.

New policies, written beginning Oct. 1, will be the first to experience the new rates and existing policies up for renewal that are eligible for a lower rate will be able to take advantage of that. Other policies that are renewed after April 1, 2022 will be subject to the new rates. Federal law requires that most rates not increase more than 18% per year.

The majority of rate changes will be fairly digestible – $20 higher or $20 lower per month. In select cases, though, the rates could increase by as much as $100 per month, a significant leap at a time when money is tight for many people. (Under the previous methodology, all policyholders saw annual increases without knowing the full risk rate of their area.)

At least one Florida homeowner, over a multiple year period, can expect to see an increase of nearly $10,000, reports the Tampa Bay Times.

The agency has released data showing how policyholders in each state will be impacted.

“The NFIP’s new rating methodology is long overdue since it hasn’t been updated in more than 40 years,” said David Maurstad, senior executive of the National Flood Insurance Program in a statement. “Now is the right time to modernize how risk is identified, priced and communicated. By doing so we empower policyholders to make informed decisions to protect their homes and businesses from life-changing flooding events that will strike in the months and years ahead due to climate change.”

More must-read business news and analysis from Fortune:

• What a modern energy crisis looks like and why no country is safe
• Frustrated carmakers upend industry after chip shortage shatters their faith in suppliers
• Portugal leads the world in COVID-19 vaccinations
• A WHO-approved Novavax vaccine could upstage mRNA jabs—if it can solve its manufacturing delays
• Bitcoin has another major pollution problem brewing