“When I started here, all there was was swamp,” Michael Palin’s scheming king declares in the 1975 comedy “Monty Python and the Holy Grail.”

Against the advice of his peers, he recounts how he built a castle there anyway. It sank into the swamp. So did the second. Ditto for the third. Though by this point he had become an expert on his kingdom’s topography, annual rainfall and susceptibility to flooding — not to mention the cost of rebuilding — the king commissioned a fourth castle. This one, he boasts without a hint of worry, has stayed up.

If City Council fails to pass Mayor Sylvester Turner’s new development rules, we’ll have embraced the kind of absurdity the Pythons mined for laughs. And our castles will keep sinking into the swamp.

The new rules are only a first step, but absolutely must be taken. They would require new construction within the 500-year floodplain to be lifted two feet above the projected flood level during a 500-year storm.

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The mayor’s plan would protect 86,000 parcels, nearly doubling the amount of plots covered under Houston’s existing rules, which only apply to homes in the 100-year floodplain. This is a big step, as dogged reporting by the Chronicle’s David Hunn, Matt Dempsey and Mihir Zaveri revealed last week that three-quarters of the homes flooded by Hurricane Harvey were outside the 100-year floodplain.

According to a city report, 84 percent of homes Harvey flooded in Houston’s floodplains would have been spared if they had been built to the new standard.

Once this plan passes, the next step must involve writing rules to protect homes outside the floodplain.

Harris County, thanks to the leadership of County Judge Ed Emmett, rushed to pass tougher construction regulations in December. We hoped City Council would quickly follow. Hurricane season begins in less than 60 days, and yet Houston drags its feet.

“No more hesitating. No more debate,” we cajoled councilmembers in mid-March. But delay they did, declining to hold a vote March 28. We were as perplexed as Mayor Turner, who noted that the new rules enjoy widespread public support.

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Developers point out that in a city that already lacks enough affordable housing, these rules will increase building costs. They make a fair point — but this eschew-regulation-to-keep-costs-down approach is what landed Houston in this problem in the first place.

The rules appear poised to pass, and we hope they do so unanimously. Houstonians deserve a strong endorsement of these rules by their elected leaders, as a weak mandate may only embolden officials down the road to loosen the regulations.

Voters should pay special attention to council members who oppose the new rules. In next year’s city elections, residents will undoubtedly have Harvey on their minds when deciding which politicians have Houston’s best interests at heart.

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With these rules on the books, city officials can focus on other steps necessary to mitigate future flooding, like creating new flood infrastructure; better equipping the Houston Fire Department to conduct water rescues; and reducing runoff, such as by eliminating minimum parking requirements. The city must also consider buying out hundreds or thousands of homes in flood zones and converting the land to green space.

Houston needs to finally start reversing the consequences of unrestrained, careless development.

If our leaders refuse to, we may well have to change City Hall’s name to Swamp Castle.