(1) In this Chapter -

"qualifying period" means the period commencing - (a) for the purposes of any provision of this Chapter other than section 334, 335 or 336, the 6th day of April, 1991, or (b) for the purposes of sections 334 to 336, the 30th day of January, 1991, and ending on - (i) the 5th day of April, 1999, or (ii) the 31st day of December, 1999, where, in relation to the construction of, conversion into, refurbishment of, or, as the case may be, construction or refurbishment of a house which is a qualifying premises within the meaning of section 334, 335, 336 or 337, the corporation of the country borough of Dublin gives a certificate in writing, on or before the 31st day of July, 1999, to the person constructing, converting or refurbishing, as the case may be, the house stating that it is satisfied that not less than 50 per cent of the total cost of the house and the site thereof had been incurred on or before the 5th day of April, 1999;