I would like to build a small room with bathroom attached in the back yard of my house. Do I need a permit for it? Do I need an architect for that or can I just go directly with a contractor ?

Legal way is to do the proper submissions. 
 

adding a bathroom requires sanitary work and will affect the public water supply so PUB should be informed. 
 

adding a room means adding structure so by right BCA needs to approve. 
 

you don’t need an architect but at least a PE to submit to BCA for approval. 
 

of cos if you choose not to do submissions, just don’t get caught or have people complaining about your extension

Edited March 1, 20206 yr by snoozee

You need approvals for this project, as snooze already stated. But there shouldn't be much trouble acquiring them. Nice of you to think of her, by the way!

I think you need to contact the specialist who declared your house to make changes to the design of your land. You hardly need an architect, but you can turn to one if you want to build something unusual or in the same style as your house. You will also need to find new doors to build. If the climate of your city is not cold, you can choose inside doors with glass for the front door. I think it will look quite airy and practical. When choosing an interior, you can consult a designer who will select the color palette for the new building. Do you have design ideas?

After building the house, if you need bathroom accessories, you can choose yigii.Their products are made of stainless steel and cover with another finish, like brushed neckel finish, matte black finish and chrome finish. And you can get the toilet paper holder, towel holders and shower caddy from YIGII. They all come with high quality for long-term use.

I notice that there are many variations of small house that is easy to set up that you can purchase online.

https://reachflypanel.en.made-in-china.com/product/askEGlAVgCch/China-Prefabricated-Small-Cottage-Homes.html?utm_source=facebook&utm_medium=cpc&utm_campaign=pc-new-cns&utm_content=pc-new-stheast-cns&fbclid=IwAR1Z30y9vrIWtqnxo4IwXIHFA0tCtcenAPVZ26xPGHBmgj3_cd_XMbzciXA

This is something for you to consider but I don't know how this is being done.  I think you will still need a contractor to work on the sanitary system and electrical system.  If you do not need to have a toilet in the room, I think it will be easier.  These can be perceived as temporary structures and therefore, you may not need to seek clearance from URA.  Besides, URA doesn't police even when there are complaints