In simple terms lenders will often lend against the value of the land as well as up to 100% of the construction costs. Funds for the development works are normally paid in arrears so it’s prudent to have a percentage of the build costs in hand.
As an example, a £1m building project might need 10 ‘drawdowns’ of finance from the lender, so you would need to be fronting the £100k of costs until the lender releases the next tranch to you.
If you’ve only got £200k worth of total build costs then it’s not viable for the lender to offer 10 drawdowns of £20k, so you will need more than the standard 10-15% of construction costs as cashflow.