Offshore Mahakam Field development has reached a mature level. With almost 400 wells occupy all platforms fully. Finding target for new wells with adequate reserves to pass the economic cut-off limit has been a challenging task. Multiple reservoirs for a re-entry (side-track) from existing well in Offshore Mahakam field become a base case by targeting both shallow reservoirs with sand control requirement and deeper reservoirs at consolidated sandstone. Conventionally, side-track wells are drilled in two separated section to allow Gravel Pack completion inside 9-5/8" casing at the upper part for shallow reservoirs and then drill deeper section for tubingless completion with 4-1/2" liner. Completing these marginal wells using current conventional well architecture makes these wells uneconomical. Answering the challenge and align to cost culture program, Pertamina Hulu Mahakam launched a collaborative project aiming for cost-effective wells against well reserves. Extended drilling envelope was introduced as the outcome of well architecture simplification process. Both shallow and deep reservoirs are drilled in one long open hole section then cased and cemented with single string tapered casing and tubing. Considering sand risk production, unconsolidated shallow gas reservoirs are accommodated by gravel pack completion in the upper part. This new type of architecture has challenge related to hole stability for long open hole management and cementing strategy to ensure high quality cement sheath behind tapered string tapered casing and tubing. Then special equipment design and series of field test are performed prior to first application. This paper illustrates development this new architecture and success application result in Mahakam Offshore Field.