Hi,
This is not standard – 5k plus 15k is about standard, not 15%. They are obviously getting worried about people pulling out of purchases by the time the property is built (as property may be worth less by then). You would be crazy to pay this deposit in the current market. You would be far better advised to look at second hand and ready to go options in the same area.
i was recently waiting 24 months for an apartment that was promised in 6 months - and I had to fund the 20k deposit for that time. In the current market you would be foolish to pay 48k.