Where did the treaty restrict fortifications?

Did the Washington Treaty generally prevent Britain and the Dominions from improving naval fortifications?

No. Its restrictions applied to specified Pacific areas, while many Atlantic, Australian, Indian, and Singapore defenses remained open to investment.

Fortification restrictions applied to specified areas, mainly in the Pacific. Hong Kong presented a particular problem. Britain could still strengthen Atlantic bases, Australian defenses, India, and Singapore within the treaty framework. Inadequate bases and coastal defenses elsewhere were political funding choices rather than unavoidable products of the treaty.