Financial due diligence is the stage that can make or break an external-growth transaction or a fundraising. Done well, it protects the acquirer from nasty surprises and lets the seller defend the valuation. Poorly scoped, it slows the closing or breaks trust. This guide details the scope, methodology, costs and checklist of financial due diligence in 2026, from both the sell-side and buy-side perspectives, for France, Belgium, Switzerland and the UAE.