Determine the nominal perimeter.

The length of the room is $L = 12.0$ m and the width is $W = 8.0$ m.
The nominal perimeter $P_0$ is given by the formula $P = 2(L + W)$.

Since the measurements are given to one decimal place (e.g., 12.0 m, 8.0 m), the maximum possible absolute error for each measurement is half of the smallest unit, which is 0.1 m.
Absolute error in length, $\Delta L = \frac{0.1}{2} = 0.05$ m.
Absolute error in width, $\Delta W = \frac{0.1}{2} = 0.05$ m.

The perimeter $P = 2(L + W)$. The maximum absolute error in the perimeter, $\Delta P$, is the sum of the absolute errors in length and width, multiplied by 2.

The maximum percentage error is given by the formula:

The maximum percentage error in estimating the perimeter of the room is $\boxed{0.5\%}$.