The 12-month transitional period from 24 March 2027 to 24 March 2028 protects developments already in construction. The rules are tighter than they look — and most volume developers are choosing to design forward to FHS rather than rely on the transitional grace.
How transitional applies
A project qualifies for transitional protection if a valid Building Control application (Full Plans or Building Notice) has been submitted before 24 March 2027 AND meaningful work has commenced on site. Submitting paperwork without starting construction does not preserve the transitional protection.
What "meaningful work" means
The Approved Document language is "commenced" which is interpreted by Building Control bodies as evidence of substantive site activity — typically foundations and slab. Site clearance and groundworks alone are insufficient.
The plot-level test
For multi-plot developments, the transitional test is applied per plot. Plots not commenced by 24 March 2027 must be built to FHS standards even if other plots on the same development qualify for transition. This matters most on slow-moving sites: a 200-plot scheme with 60 plots commenced will see 140 built to FHS.
Why most volume developers are designing forward
Operationally, running two specifications in parallel is expensive. Most Top-20 housebuilders are designing their 2027+ phases to FHS spec from now, even where transitional protection technically applies. Marketing also matters: buyers in 2027 expect FHS-spec homes.
Practical advice for self-builders
If you can complete Building Control approval and start groundworks before 24 March 2027, you have flexibility. But the FHS specification is what buyers will want from 2027 onward — and the marginal cost is small. Most architects are designing self-builds to FHS from the outset.