22 to 25k initially would be my preference. Don't agree with 20k max as that's our limit... to me that may be 'realistic' (but highly debatable) but is borderline limited, unambitious, 'tinpot' mentality.... something I think Bill is looking to shed. Limited, unambitious thinking can then transpire to fabric and ethos of the future of the club.... and only serves to potentially limit growth, aspiration and overall message it sends imo.
Obviously if 25k is much more likely to get rejected, whereas 20k pretty is assured of getting the green light, then 20k makes sense... or other complications that may hinder it happening at all.
Personally I'd much rather take risk of things not going to plan and having to play in a half empty stadium due to relegation and prolongex lower league football, than be too cautious and end up restricted by a cramped stadium, with loads still not able to get in a few years down the line.
The appetite and catchment absolutely is there in Dorset imo, to fill 25k each week. I think Bill is the real deal, and we're on the cusp of something way beyond what we comprehended a decade or so back.
Whether it's 20 or 25k, I think scope for taking capacity over 30k relatively easily will be factored into the architecture.
I also hope the ground comes with some kind of 'flair' character and differentiating features, rather than a basic, identikit bowl that looks like a load of other grounds and is built on a relative shoestring and constructed from lego/mechano. If we're going to build a new structure, I hope we spend a little more ensuring a decent capacity and quality setup that makes us proud... if we're going to build something from scratch, let's do it properly. Not a slightly bigger version of what we pretty much have now.
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Obviously its not my money, but those are my thoughts and hopes around the project.