General Strength of a Bhava

In a horoscope, to assess the results of a house with respect to areas it signifies, we need to look at influence of other planets on these three elements -
1. The house,
2. Influence on the lord of the house, and
3. Influence on the Karaka of the house.
The karaka of the house is not the karaka for all purposes. The trait of the house is important here, e.g. Sun is a karak for father and for recognition from king. In a horoscope, father is looked at from 9th house and recognition from state is looked at from 10th house. This means that when we look at these traits, we would look at Sun in both cases. Similarly, vehicles and houses are judged from fourth bhava. But for vehicles we look at Venus and for houses we need to look at Mars. Mars and Venus are grihakaraka and vahanakaraka respectively.
The influence could be through occupation of the house by a planet and through drishti or combination of planets with the lord or karaka. Beneficial influence on these elements contributes to their strength. If all three elements are strong, then 100% results for the house can be obtained. Correspondingly, malefic influences reduce the strength of the house and are harmful for the results being signified by the house.
Strength of a house in a horoscope
An influence on the house should be seen from drishti or occupation of the house by other planets. Beneficial influence comes from occupation or full aspect of planets like Jupiter, Venus, Moon and planets in exaltation. Malefic influence comes from (generally speaking) Saturn, Mars, Rahu/Ketu and (specifically) lords of 6th, 8th and 12th houses. A house in a horoscope is considered weak if its lord is located 6th, 8th or 12th from the house. If Maraka planets are co-located in a house, it is considered to harmful to the aspect of the house during their transits and dasha period.
Strength of lord or Karaka
To look at the strength of lord or the karaka of the house, general rules for planetary strength in a horoscope are applicable, i.e. a planet is considered strong if:
• it is in exaltation,
• situated in its own rashi or in a rashi of a friendly planet,
• in kendra or trikona in a horoscope,
• is vargottama,
• close to the mid-point of a bhava,
• influenced by aspect from beneficial planets,
• influenced by beneficial planets by being situated in the same house in a horoscope.
Further, a planet in a horoscope is considered weak if:
• it is in debilitation,
• situated in a rashi of a planet inimical to it,
• situated in 6th, 8th or 12 house in a horoscope,
• by being located at bhava sandhi,
• influenced by aspect from malefic planets,
• influenced by malefic planets by being situated in the same house in a horoscope, or
• if it is combust by being close to the Sun.
According to Uttar Kalamrita, if the lord of a house in a horoscope is governed by any of the above rules of debilitation, even the aspect of beneficial planets cannot rescue it. There are, however, exceptions to these rule. Location of Jupiter in 8th house cannot be considered bad as by being situated there, it can influence 2nd and 4th house through its 7th and 9th full drishti. Venus is considered a yogakaraka if it is situated in 6th or 12th house.
If the lords of 6th, 8th or 12th house are located in one of these inimical houses, it is considered to be auspicious and gives rise to viprit rajyoga.