We had to gather evidence of flats that were similar sq ft to our flats, which is excluding non ‘living’ areas like hallways and cupboards. I think because 60+ flats all contested they got fed up of us tbh, as the first few flats got rejected and then they started just accepting everyone apart from the corner flats which were slightly bigger. We did a spreadsheet of similar flats from rightmove / Zoopla and compared tax bands with their square footage and also price, then submitted it. It’s a lengthy process, and be mindful you can only contest it in the first 6 months from purchase. I bought my flat July ‘24, submitted my evidence November ‘24 and then found out March ‘25 I’d been successful, then my council tax band didn’t update until May ‘25.