Parking is available free outside the shop on the high street, however this is a one hour max stay. 

If booking a longer treatment you can find a pay and display car park at:

St Martin's Rd, Knebworth SG3 6DD (charged at £0.45/ hour pay and display). 

or if you don't mind a short walk & can find a space there is 2 hour free parking at:

Milestone Rd, Knebworth SG3 6DA

Free unrestricted parking can also be found a little further away on London Road just out of the high street toward Welwyn direction.