You must apply and be approved before you travel. Airlines will refuse to board you without it, and applying takes time — start well before you book anything you cannot change.
Status
Visa required
Stay permitted
Single entry. Enter within 60 days of approval; stay up to 30 days.
Changing planes: a passport from Iran needs a visa even to stay
airside in United Kingdom and Schengen area airports, unless you hold an exempting
document. Transit ruleslegislation.gov.uk · read eur-lex.europa.eu · read
What Laos asks for
Passport valid for more than 180 days from arrival
Scan of the passport bio page, including the two MRZ lines
Photograph 4x6 cm, taken within the last 6 months
The eVisa Approval Letter, shown on arrival
Worth knowing
The eVisa fee is not refundable.
The eVisa can be used only at the airports, bridges and railway stations the site lists as eVisa ports.
Carry the Approval Letter. Without it you will be refused entry.
How long it takes
Apply no more than 60 days and no less than 5 days before you arrive.