How do I bathroom / kitchen / extension? etc.

as a percentage of the total conversion cost

But surely we aren't talking about total % savings? We're talking about % savings on the difference between a DMI/TMH loft vs lofts4U.

So even taking off a few £k here and there makes a difference, as does spreading the cost.

As I said before with inflation and rates rising I think a two pronged approach of cancelling the bling accessories/purity of materials plus borrowing would be the way to go.

But it's all dependent on circumstances. For eg we would have struggled to borrow money after our first was born based on our work situation, whereas now we could. Likewise if you're someone who can increase their hours and get paid for it, then adding 20 or 30hrs to your week will probably be better vfm than DIYing those hours at night.