I agree this situation is not balanced, but I don't like the proposed solution.
Let's look at the reality. First, if someone knows he might be attacked, he has the opportunity to prepare some nasty surprises for the attacker. The attacker can too, true, especially right at the start (surprise) but for most time it'll be the defender who sets out the rules of engagement, especially if they're on the same technological level (as in this game). So, in a combat the attacker should have strong advantage over the defender in the first round but the balance should be quickly shifted to the defender, and stay there. During the whole fight, innocents might get wounded or killed; this includes clients, girls, bartenders, bystanders, etc.
Second, if you get attacked, the police comes to investigate. They will not be too interested in defending you, should you be attacked again, but they will defend themselves, and will stay there for a while; anybody attacking during this time 1) will face a bit stronger and trained opposition, and 2) risks being jailed. On the other hand, the normal function of the building will be suspended for the time being, so you earn nothing.
Third, if the police comes, they're likely to keep a few of your guards for a while to testify. They'll heal them, if needed, but they're not going to release them soon. So, if you need to still be able to defend, you should have some spare guards.
Fourth, the investigation can result in one or both sides being jailed. The attacker can get jailed if there's evidence pointing to him, like video recording or wounded left in place. The defender can wiggle out of the unfair gun use (if he can prove he was not an aggressor, like via video recording) but can still face charges if the building's real use is discovered;
Translating this to game terms:
1) first round of combat the advantage belongs to the attacker, for the remaining rounds the advantage is to the defender. The exact amount of advantage might for example depend on building upgrades (think hidden rooms with guards, reinforced doors, or centralised door control);
2) an attack, whether successful or not, has a strong chance to trigger police investigation on the site. The police investigation takes some time (4h?); during this time anybody attacking this building faces stronger opposition, plus risks being jailed on the spot; this protection does not cover other buildings of the same player; you get no income from a building which is being investigated. You can bribe the police to go away (CC?) but you lose any protection they offer;
3) when the investigation ends, your guards are healthy but kept in the prison (for a day?), possibly some of your girls are kept too as witnesses (this should be random, and generally shorter than the guards arrest); you can bail them out (with money after half time or with CC instantly), the building is working, and if you have some idle guards, they can take up the posts in the building, best first.
What do you think?