I would definitely say a 25 foot, I often have mine fully extended over long conservatories or flat roofs, 3 storey new build town houses a must too, I often wish I had gone for 30 foot instead as I am often stretching on 3 story town houses. But 18 foot would do for 90 % of the jobs, But still need that extra on others.
I think my 5 yr old extel is about to die. Fed up of glueing it all the timeI use a 13 ft and a 23 ft . Will a gardiners 23ft be ok instead of 2 poles ? Also how do you stop grit getting up it all the time . It's doing my head in