The cynical side of me just sees this as another opportunity to up the price of a house or rental property. Basically, it will be price gouged - £4000 additional cost to the builder? That will be £15000 added to the purchase price of the house.
Speak from experience with my own older house. It was built in 1994, has large south facing roof and a staggering £11K to 19K install cost for solar depending on whether or not I want batteries. I'd save £240 a year off my electricity bill as the array would never fully displace grid supply with the UK's climate (It rains A LOT here).
Hardly an incentive with a 45 year break even point on outlay at £11K. This is why uptake is so low here.