Plans modified by the government would see Brighton & Hove expand, and West Sussex divided in two.
Ricky Ryder denies intending to endanger life when he set fire to Peacehaven Mosque in October.
Two others were previously charged by Sussex Police over the attack at Peacehaven Mosque in October.
Peacehaven Mosque will open its doors to the wider community for an open day on Saturday.
The pair were in Peacehaven Mosque, in East Sussex, when the blaze began, the Old Bailey was told.
After killing Josh and Chloe Bashford, Derek Martin met the couple's children and took them out for a meal.
Counter Terrorism Policing South East says it will now lead the fire investigation in Peacehaven.
The pet's owners say they are devastated by his loss and want tougher rules brought in.
Mark Ward, from Peacehaven, was 14 when he was diagnosed with HIV and hepatitis C.
