Aniston, 44, is planning a lavish birthday party to celebrate Theroux's 43 birthday Aug 10. She has invited guests, but friends are speculating if it will turn out to be a wedding, reports dailystar.co.uk.
"They’ve hired planners and caterers and the house looks spectacular covered with white drapes," a source said.
“A huge tiered cake is on order and Jen has invited all their friends and family over to celebrate. Friends think they’ll surprise everyone by getting hitched," the source added.
The couple got engaged in August last year.