Sea Lion required visible prewar amphibious investment

When would Germany need to begin serious preparations for a viable Operation Sea Lion, and how would Britain react?

Planning and shipbuilding would need to begin around 1934 or 1935, and Britain would become deeply concerned by the visible amphibious buildup.

Germany would need to begin planning and specialized construction around 1934 or 1935. A credible invasion required purpose-built vessels and an established amphibious organization rather than improvised barges assembled after France fell. A programme large enough to make Sea Lion viable would be difficult to conceal. Britain would gain years of warning and could shape naval, air, and coastal countermeasures around the visible threat.