Caution
A sliding sash drops shut on its own, or will not lift at all, because a cord has parted inside the box frame.
Low risk
The door leans out of the frame, catches the floor on the handle side, and the gap along the top runs at an angle.
The door closes but springs back open, or you have to lean on it before the latch clicks.
Timber in a sash window has gone soft: usually the bottom rail of the lower sash or the cill below it.
The bottom few inches of an external door frame have gone soft, spongy or crumbly.
One or two treads announce themselves every time anyone uses the stairs.
High risk
The handrail or the spindles move when you lean on them, or a spindle has come out at one end.
The timber cill under a window has gone soft, dark and spongy, and a screwdriver pushes into it.
Small round holes in floorboards, joists or roof timbers, sometimes with fine dust beneath them.
A step moves, tilts or drops slightly when you stand on it, or the front edge has split away.
Describe the job and get a fixed price from a local carpenter.