The first converted Bunnings was also in St Albans. Hemel Hempstead and Milton Keynes are due to be converted by 30 June.
The store is 3,700sqm. There is an indoor children's playground, timber cutting, colour wall, engraving and key cutting, as well as DIY and garden centre.
There are 53 new jobs full and part-time making 71 staff in all.
The store has 90 brands featured across 24,000 lines and an activity organiser for community involvement.
Bunnings parent Wesfarmers results later this month could give a clearer indication of performance of the UK operation, bought in February 2016, and which made a £28m loss in its first returns announced in February.