Aberdeen-based RB Wilson Electrical had been contracted by Aberdeenshire Council to rewire and install new heating systems in 50 of its properties last year.
Four electricians arrived at 82-year-old Emily Begg's bungalow on 19 February 2018 to carry out the work, which involved accessing areas underneath the floor via hatches.
On 20 February, one of the hatches was left open and uncovered while an employee went to fetch his tools from another room.
Begg fell through the hole and sustained a deep cut on her leg, as well as severe bruising.
Her brother-in-law fell through the hatch later that day when he entered the property to check on her,
The Press and Journal reported.
The Health and Safety Executive (HSE) said both incidents could have been prevented had adequate barriers and physical warning signs been erected around the hole.
R.B. Wilson Electrical admitted breaching reg 6(3) of the Work at Height Regulations and was fined £24,000 last week at Aberdeen Sheriff Court.
HSE inspector Elaine McAllister said: "This incident could so easily have been avoided by simply carrying out correct control measures and safe working practices."