Problem with sscanf reading multiple strings from a input file

Reading the line from the file into a string via fgets() is a good first step as OP has done.

Can "House naming" include digits? like "Stdio 54"?
Yea, it can include digits and can not. If I am right, the task says nothing about namings.

The next part is tricky as there is not a unique separator between the house name and the following 3 integers.

One approach would be to find the 3 trailing integers and then let the remaining beginning text as the house name.

  while (fgets(buff, sizeof(buff), fi) != NULL) {
    int address, age, power;
    char *p = buff + strlen(buff);  // start as buff end
    p = reverse_scan_int(buff, p, &power);
    p = reverse_scan_int(buff, p, &age);
    p = reverse_scan_int(buff, p, &address);
    if (p) {
      *p = '\0';
      trim(buff);  // remove leading trailing white-space
      printf("house_name:'%s', address:%d age:%d power:%d\n", buff, address,
          age, power);
    } else {
      printf("Failed to parse '%s'\n", buff);
    }
  }

Now all we need is reverse_scan_int(). Sample untested code idea:

#include <ctype.h>
#include <stdbool.h>
char *reverse_scan_int(const char *begin, char *one_past, int *i) {
  if (one_past == NULL) {
    return NULL;
  }
  // is...() functions work best with unsigned char
  unsigned char *p = (unsigned char *) one_past;
  // Skip trailing whitespace;
  while (p > begin && isspace(p[-1])) {
    p--;
  }
  // Skip digits;
  bool digit_found = false;
  while (p > begin && isdigit(p[-1])) {
    p--;
    digit_found = true;
  }
  if (!digit_found)
    return NULL;
  if (p > begin && (p[-1] == '-' || p[-1] == '+')) {
    p--;
  }
  *i = atoi(p); // More roubust code would use `strtol()`, leave for OP.
  return (char *) p;
}

There are lots of ways to trim a string including this one.

Your problem is tricky to solve with sscanf() because there is no explicit separator between the house name and the 3 numeric fields. %s is inappropriate: it parses a single word. In your program, sscanf() actually fails to convert the numbers and returns 1 for all lines, leading to undefined behavior when you compare the numeric values that are actually uninitialized.

Here is a modified version using the %[ conversion specification:

#include <stdio.h>
#include <stdlib.h>

#define F_INPUT  "input.txt"
#define EXIT_INPUT_FAIL  1

int main(void) {
    char buff[256];
    char building[100];
    int addr, age, kwh;
    FILE *fi;

    // opening the files and checking if it succeeded
    fi = fopen(F_INPUT, "r");
    if (fi == NULL) {
        printf("Error opening input file \"%s\"", F_INPUT);
        exit(EXIT_INPUT_FAIL);
    }
    while (fgets(buff, sizeof(buff), fi) != NULL) {
        /* parse the building name upto and excluding any digit,
           then accept 3 integral numbers for the address, age and power */
        if (sscanf(buff, "%99[^0-9]%d%d%d", building, &addr, &age, &kwh) != 4) {
            printf("parsing error: %s", buff);
            continue;
        }
        if (kwh < 200) {
            puts(buff);
            printf("Sustainable\n");
        } else
        if (kwh < 300 && age < 40) {
            puts(buff);
            printf("Needs renovation\n");
        } else
        if (kwh > 300 && age > 40) {
            puts(buff);
            printf("IN DEMOLITION LIST\n");
        }
        // allocate structure with building details and append it to the list or array of buildings
    }
    /* close the files when they're not needed anymore */
    fclose(fi);
    // sort the list or array and print it
    // free the list or array
    return 0;
}