Renting changes the order of operations. Instead of asking how to remake the architecture, ask what you can bring into the room, use properly and carry out again later.
That constraint is not necessarily bad for Cottage Goth. Rugs, lamps, old furniture, books, curtains and art are exactly the things that make a room feel accumulated rather than themed.
Start with the light because it changes everything else
A rented room with white walls can still feel warm and shadowed if the main light is no longer doing all the work. Add table lamps, floor lamps and plug-in wall lights where you sit, read and work.
Use warm bulbs and shades that direct light rather than flooding the ceiling. The white walls then become quiet reflectors instead of the dominant feature.

Put colour on things you own
A large rug, dark curtains, a folding screen, painted freestanding bookcase or upholstered chair can introduce deep green, brown, oxblood and black without touching the landlord's paint.
Curtains are particularly powerful. Store the originals safely if you replace them, and check how the pole or track can be used without damage.
Old furniture is more useful than fake architecture
Do not spend money trying to imitate beams or panelling temporarily. One old cabinet, table, chest or mirror has genuine material presence and comes with you when you leave.

Rugs can change a modern floor dramatically
Laminate and pale carpet often dominate rental rooms because they cover so much area. A large wool or patterned rug breaks that field and gives furniture something to sit on visually.
Choose a rug for the dimensions of the room, not merely the motif. A generous faded rug usually does more than several small decorative mats.
Art and mirrors build vertical interest
If you can use picture hooks, hang real frames. If you cannot, use existing fixings, picture rails, shelves or freestanding furniture rather than trusting heavy antique mirrors to adhesive strips.
Botanical prints, landscape paintings, old photographs and mirrors can create the vertical rhythm that panelling or architectural detail would otherwise provide.

Peel-and-stick is not automatically harmless
Removable wallpaper and vinyl can lift paint, trap moisture or leave adhesive. Test products in an inconspicuous place and read the tenancy agreement before treating “removable” as a guarantee.
In kitchens and bathrooms, heat and steam make temporary finishes even less predictable. Portable objects are often the safer investment.
Let ordinary life remain visible
A rental becomes convincing when books, coats, plants, mugs and chargers settle into the aesthetic rather than being hidden for photographs. Baskets, trays, cabinets and hooks can organise without sterilising the room.

Spend on what survives the move
A good lamp, rug, wool blanket, chair, mirror or bookcase can move through several homes. That makes them better long-term purchases than temporary imitation mouldings or novelty decor.
The rental version of Cottage Goth is therefore unusually honest: the architecture may be modern, but the things you live with can still have texture, age and shadow.