Is it permissible for me who works as an architect to design places of sin or houses of worship for non-Muslims?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Every matter that is performed by a believer can change to be an act of worship according to the intention. Touching on the topic of occupation, it is a part of life. Thus, work is considered worship when one set the intention of doing it only for the sake of Allah with sincerity and hope towards Him.
From Abu Hurairah RA, Rasullullah PBUH said:
مَنْ طَلَبَ الدُّنْيَا حَلَالَا اسْتِعْفَافًا عَنِ الْمَسْأَلَةِ ، وَسَعْيَا عَلَى أَهْلِهِ ، وَتَعَطُّفًا عَلَى جَارِهِ ، جَاءَ يَوْمَ الْقِيَامَةِ وَوَجْهُهُ كَالْقَمَرِ لَيْلَةَ الْبَدْرِ
“Whoever seeks sustenance in order to protect himself from begging and with the intention of fulfilling his family’s needs as well as helping his neighbours, surely he’ll come in the hereafter with a face shining like the full moon.” 
In answering this issue, we detail the above question into several matters as follows:
First: The Ruling of Working for Non-Muslims
In discussing this issue, the majority of jurists state that it is permissible. Most of them based their opinion on a hadith from Ali bin Abi Thalib RA, where he said:
فَمَرَرْتُ بِيَهُودِيٍّ فِي مَالٍ لَهُ وَهُوَ يَسْقِي بِبَكَرَةٍ لَهُ فَاطَّلَعْتُ عَلَيْهِ مِنْ ثُلْمَةٍ فِي الْحَائِطِ فَقَالَ مَا لَكَ يَا أَعْرَابِيُّ هَلْ لَكَ فِي كُلِّ دَلْوٍ بِتَمْرَةٍ قُلْتُ نَعَمْ فَافْتَحِ الْبَابَ حَتَّى أَدْخُلَ فَفَتَحَ فَدَخَلْتُ فَأَعْطَانِي دَلْوَهُ فَكُلَّمَا نَزَعْتُ دَلْوًا أَعْطَانِي تَمْرَةً حَتَّى إِذَا امْتَلأَتْ كَفِّي أَرْسَلْتُ دَلْوَهُ وَقُلْتُ حَسْبِي فَأَكَلْتُهَا ثُمَّ جَرَعْتُ مِنَ الْمَاءِ فَشَرِبْتُ ثُمَّ جِئْتُ الْمَسْجِدَ فَوَجَدْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فِيهِ
“I went in search of something. I passed by a Jew on his property drawing water (from a well) with a pulley. I watched him from a gap in the fence. He said: “what is wrong with you O Arab! Would you like to get a date for every bucket?” I said: “Yes. Open the door so I can come in.” He opened the door, I entered and he gave me his bucket. Then for every bucket I pulled out, he would give me a date, until when it was enough for me. I put his bucket down and said: “I think I had enough to eat” then I scooped some water to drink it. Then I came to the Masjid and found the Messenger of Allah PBUH in it.'” 
According to the above narration, it is permissible for a Muslim to work for a non-Muslim in order to get paid for the services he provided.
Second: The Ruling of Helping a Non-Muslim
In the issue of a Muslim helping a non-Muslim, the type of help offered is divided into two:
- Social welfare
In the issue of social welfare, we as Muslims are commanded to do good with any human being regardless of the differences in religion, race, lineage, skin color and others. This is based on a narration of Abu Dzar RA, where the Messenger PBUH said:
وَخَالِقِ النَّاسَ بِخُلُقٍ حَسَنٍ
“And behave decently towards people.” 
The word people in the above hadith is considered as a general phrase that covers all people, regardless of who they are, including the disbelievers. Thus, it is obligatory to do good and interact with Muslims admirably.
The same is stated in the following statement of Allah SWT:
لَّا يَنْهَاكُمُ اللَّهُ عَنِ الَّذِينَ لَمْ يُقَاتِلُوكُمْ فِي الدِّينِ وَلَمْ يُخْرِجُوكُم مِّن دِيَارِكُمْ أَن تَبَرُّوهُمْ وَتُقْسِطُوا إِلَيْهِمْ ۚ إِنَّ اللَّهَ يُحِبُّ الْمُقْسِطِينَ
“Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes – from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly.” 
This verse clearly shows that the ruling of doing good and being charitable to a non-Muslim group of people who aren’t against the Muslims is permissible in syarak.
For example, a Muslim who sees a non-Muslim who is hungry or thirsty, then it is commanded for him to help. Likewise, if a Muslim sees a blind non-Muslim wanting to cross the road, he should help the non-Muslim in this situation. This showcases the blessing of Islam for the whole universe.
- Worship in another religion and transgression
In the issue which involves other religions’ acts of worship, it is impermissible for a Muslim to help, for it involves shirk towards Allah SWT. This is clear according to the statement of Allah SWT:
وَلَا تَعَاوَنُوا عَلَى الْإِثْمِ وَالْعُدْوَانِ
“And do not cooperate in sin and aggression.” 
In this matter, it clearly shows that Allah SWT prohibited help in any sinful matters and also all the matters that transgress what has been set by Allah SWT.
To close, we state that the ruling of working with a company whose leader is a non-Muslim is generally permissible. However, if the work includes prohibited matters such as helping to harm the Muslim ummah, making wine, helping towards shirk and transgression is prohibited.
Referring to the question presented, we state that such matters should be avoided. The reason anything that leads towards a bad or prohibited purpose is also considered as such. This is as stated in the Islamic legal maxim:
الْوَسَائِلُ لَهَا أَحْكَام الْمَقَاصِد
“Methodologies have the same ruling as their purpose.”
Hence, designing places where sins towards Allah SWT are going to be committed is prohibited.
We suggest that you discuss with your employer wisely and explain this situation to get the best resolution for both parties. Hopefully, Allah SWT will grant us all the correct understanding of this religion. Amin.