としたら details
Examples
supposing that; if we assume — a hypothesis the speaker sets up for the sake of argument, often counterfactual or unlikely
もし宝くじが当たったとしたら、何をする？
moshi takarakuji ga atatta to shitara, nani o suru?
If you won the lottery, what would you do? casual
この計画が本当に実現するとしたら、予算はどのくらい必要ですか。
kono keikaku ga hontou ni jitsugen suru to shitara, yosan wa dono kurai hitsuyou desu ka.
Supposing this plan actually happened, about how much budget would we need? neutral
仮に交渉が決裂するとすれば、次の一手を今から考えておくべきだ。
kari ni koushou ga ketsuretsu suru to sureba, tsugi no itte o ima kara kangaete oku beki da.
Supposing the negotiations were to collapse, we should be thinking now about our next move. formal
Formation
plain clause + としたらattaches to plain clause
Notes
する marking is what licenses the overt-supposition reading, distinct from
なら's anaphoric pickup of a premise already on the table.
Related patterns
Similar
なら is anaphoric — it takes up a premise already on the table, something said, assumed, or contextually given; としたら overtly marks its clause as a constructed supposition via する, which is why it reads naturally heading into argumentation and into questions probing a hypothetical (この計画[けいかく]が実現[じつげん]するとしたら、予算[よさん]はどのくらい必要[ひつよう]ですか).
としたら proposes a hypothesis the speaker builds for the sake of argument, often counterfactual; 場合 states a factual branch a rule or plan needs to cover, and reads far more neutral and procedural.
Both add とする to mark an overt, constructed supposition, but としたら threads into a CONDITIONAL continuation (if X, then Y follows — 予算[よさん]が足[た]りないとしたら、どうする？), while としても threads into a CONCESSION (even if X, Y still doesn't change — 予算[よさん]が足[た]りないとしても、進[すす]めるべきだ) — same hypothesis frame, opposite logical connective riding on it.
Lookalikes
The と here is not the automatic-consequence と of 押[お]すと — としたら needs the full と＋する＋たら frame to mark a stated hypothesis and drops と's lawlike, exceptionless reading; 明日[あした]雨[あめ]が降[ふ]るとしたら imagines a scenario, 雨[あめ]が降[ふ]ると states a rule.