Can you get the builder to give you a lump sum quote inclusive of VAT..
Ie. £20,000 Incl. of VAT.
As he has to show the VAT being charged his invoice is either
£19,047.62 + ECO VAT @ 5% ( £952.38 ) = £20,000
Or
£16,666.66 + Vat @ 20% ( £3,333.34 ) = £20,000