I have a base 30 m.Dem . However, i also have 1m Dem for an area of interest only. I would like to overlay the 1m Dem on the 30 m Dem meaning the area of interest will have 1m and the rest 30m.
Is that possible?
A raster file cannot have multiple resolutions. So if you want a mosaic of 30m and 1m resolution raster, you will need to build a 1 m resolution raster. Over a large area, this might be huge (1 m resolution is 900 times larger than 30m resolution), and based on your comments it is approximately 1 Tb in your case (large but manageable).
Here are some tips to solve your problem, but more details about the purpose of the analysis would help you select the best option :
Conceptual solution: use TIN instead of raster, because TIN have irregular sizes
Storage solution : By default, your DEM will be in floating points, but do you need it ? You could divide the size by 2 or 4 if you could store it in int16 or in Bytes. Storing in Bytes is probably too coarse for a DEM, but you could multiply your heigth by 10 and store it in uint16 (just be careful if you have negative elevation (e.g. Dead Sea, then use int16 instead of uint16) or mountains above 6500 m (e.g Everest, then shift your elevation by 3000m). Another solution is to make a compromise: using a 2 meter final resolution help you dividind the size of your dataset by 4. Finally, make sure that you also tick the advanced parameters and add a compression, which would help reducing the size of your dataset.
Virtual file solution: building a vrt will make sure that you don't occupy too much disk space. The resampling to one meter could be done on the fly with a -r cubic option. A simple trick if your 1 m raster is a rectangle in the middile of your 30 rasters is to create 4 vrts at 30 m that surround your 1 m raster, the merge those four 30m vrt's with the 1m vrt (-tr 1 1 in the options to set the final resolution at 1 meter)