Häfner was speaking at a press conference in Strasbourg, alongside fellow Greens/EFA deputy Ulrike Lunacek and new chair of the single seat intergroup Anna Maria Corazza Bildt, a member of the EPP group.
According to a recent study, commissioned by several members of the Green party, an "overwhelming majority" of citizens across Europe support the notion of a single seat, making it a matter of "democracy", explained Häfner.
"Eighty per cent of our legislative activities happen in trilogue the whole year round, and how can that operate if parliament is regularly away for a week" - Gerald Häfner
The study, which took place in Germany, France, Italy, Sweden and Poland, found that France was most in favour of a single seat, with 83 per cent of citizens agreeing with proposals, while Poland were the least enthusiastic with 58 per cent.
However, those against the proposals ranged from just 12 per cent in Germany to 18 per cent in Poland.
"Eighty per cent of our legislative activities happen in trilogue the whole year round, and how can that operate if parliament is regularly away for a week," asked the German MEP.
In addition, he pointed to the fact that "Strasbourg is only used for 11 per cent of the year".
Parliament has to "think differently", he said, adding, "parliament alone should decide where and when it sits, not the [European] council".
"The parliament and council are equal as a co-legislators," explained Häfner.
He said that asking for a treaty change to allow for a single seat is "not about France, or Strasbourg" but "is a question of doing something that is valuable and substantial".
"In democracies we treaty decisions, but in the long run if we see that something is rather ridiculous we need to be able to change. We want the parliament to be even more democratic… for its citizens," he concluded.
Meanwhile, Lunacek, who was recently elected as a vice-president of parliament, said that the results of the poll are "very encouraging" and that "it is extremely important that this study has been done".
"[The single seat] is a campaign for something, not against it, in favour of democracy" - Anna Maria Corazza Bildt
"We have asked for the parliament to be able to decide ourselves, this is a democratic right", she argued.
"In these elections, groups and MEPs were elected to a parliament that they don't want to have. I think we have the task, as pro-Europeans, to work for the citizens so that they really feel represented by us", she stressed.
Corazza Bildt said that the study "gives us the motivation to continue our campaign", adding, "This is a campaign for something, not against it, in favour of democracy".
"We have committed ourselves to putting forward a proposal", said the Swedish deputy, adding, "We want to continue to respect what parliament has already voted on."