Garrett's first mission is, of course, to gain access into the Architect's estate, which means that you will have to make through the garden around it [Eastwick's Grand House - Garden]. This entails certain obstacles, however, and this means, at least, five guards that are patrolling this area. Two of the enemies are occupying the main square, but you should be even more concerned about the two opponents that patrol the entire garden together, especially that one of them has a torch. Apart from that, you need to watch out for the guard with the crossbow, on the balcony above the entrance. Therefore, you should avoid standing in well-lit places and being spotted. Note - one of the opponents on the main square has a Coin Purse - 32G, but I recommend against taking the risk connected with robbing him.
The guards are not your only problem, because there are dog cages on both the eastern and the Western side of the garden, which may quickly alarm the enemies. Exceptionally in this case, I recommend that you use swoop to avoid raising the dogs' attention here, especially that you will have to remember about the abovementioned opponent with a torch. Garrett will not be able to hide from him effectively (nor stay in one place for longer, as a result) because there are no side-alleys in the garden, which would allow you to wait for the patrolman to leave the area.
You can infiltrate the Architect's villa in four different ways, which have also been presented on the map. Variant A assumes that you enter through the main door. This is not the worst idea, but you need to be mindful of actions of all the guards. The best way to reach the entrance is through the Western or the eastern part of the garden, which will allow you to walk around the well-lit square. Towards the end, you could use the swoop to your aid. After you open the main door of the estate, you will gain access to the ground floor of the building [Eastwick's Grand House - First Floor].
Variant B assumes that you reach the door on the upper balcony, directly above the main entrance. You can reach this balcony in two different ways - in the North-Western part of the garden, there is an interactive catch, at which you can fire the rope arrow. In the North-Eastern part of the garden, on the other hand, there is a ladder that you can lower by shooting an arrow at the ladder's blockade. Regardless of which path you take, watch out for the opponent with the crossbow, who is patrolling the balcony. It is for the best to wait until this guard walks away, and sneak past him. After you open the door on the balcony, you will enter the estate's second floor [Eastwick's Grand House - Second Floor]. Note -The guard on the balcony has a Coin Purse - 9G attached to his belt, although it is quite risky to rob him.
Variant C is a clever one and it assumes that you take the scaffolding in the North-Eastern part of the garden. The best way to reach the scaffolding is over the abovementioned ladder that you lower. Start climbing over the element s of the scaffolding and reach a small tunnel, through which you can squeeze. The linear walk along the tunnel will take you to the hole. After you jump inside, you will end up in the bedroom at the villa's second floor [Eastwick's Grand House - Second Floor].
The final variant D is relatively risky, which does not mean that you should ignore it. In the Western part of the garden, there is a small cottage, whose door is, unfortunately, locked. You can wait for the nearby lamp to go off, or stand more to the right and hope that Garrett will not be illuminated. Quickly, use the lockpicks to open the door (I recommend that you use the Focus to your aid, if possible). In the meantime, remember about the nearby guards, and especially about the opponent with a torch, who may illuminate the cottage's area unexpectedly.
After you get into the cottage, find the Poppy, and interact with the valve here, which will result in it being shut. You can now locate the button shown in the above screenshot and press it, thanks to which you will enter a secret chamber. There is a ladder here, which will take to the underground passage neighboring the main part of the cellar. [Eastwick's Grand House - Underground Passage].